Dear J.,
First, please know that I just responded not two weeks ago to a similar query from a mom in the same position, with the same questions and concerns, so you are not the only one!
I think you may be able to find that question and answer by clicking on the "Read More From This Person" link at the end of this answer. It's not showing on my screen, so I'm not sure....
Basically, I tried to reassure that mom just as I am reassuring you that your son's growing interest could mean any number of things, and certainly is not an indication that you have a young cross-dresser, transsexual or anything else that you may be wondering at this point.
He may simply love his mom and be trying to adopt your tastes and interests as part of the bonding process. Does he know you like sundresses? Maybe you've made a passing comment in a store like, "Oh, I just love that cute little sundress over there!"
Remember, your son's value system is growing right along with his little feet and legs at this stage of his life, and will be shaped by what you -- his primary role model -- value. If you seem to think something is great, he is likely going to think the same thing is wonderful -- even if it's a sundress.
It could also simply be that your son enjoys dressing up, so I would certainly explore that avenue first. You might start a dress-up trunk for him, filled with items that he can assemble into fun and imaginative costumes. Put out the word to friends and family that you are collecting hand-me-down costumes and you'll probably get a few Halloween leftovers. Children's consignment shops usually carry costumes, and this is a great time of year to start looking for them.
Mark your calendar now for Nov. 2-4, which are the days to find the best costume deals of the year at the larger discount stores such as Target and Wal-mart. Halloween costumes which did not sell are marked down to pennies on the dollar on those days, so you could score a few costumes to add to the trunk then!
You've probably noticed that most boy costumes are much simpler and darker than girl costumes, so this may make them less appealing to your son. After all, if you're naturally attracted to sparkles, an Incredible Hulk costume is just not going to cut it for you! But a fancy wizard's cape might, if it was silver and sparkly, and magically swirled about you as you walked. A silver wand (even one made out of straws covered in aluminum foil and painted with silver glitter glue) might make the cape even more attractive to you.
The key here is to take fabrics and items that naturally appeal to your son and assemble them into costumes that gently and lovingly reinforce his gender identity without making him feel "wrong" for being attracted to them. You'll need to use a bit of imagination, and choose your words carefully.
Fortunately, a good story is a fun and easy way to do this, and ties in so nicely to the whole costume and make-believe idea.
Let's say, for example, that he likes your pink silk slip and wants to wear it. You might consider telling him that although boys don't usually wear pink slips, there was a time when that wasn't so....
Drape the slip around his neck,so that he can wear it and enjoy it while you talk, then snuggle up to him and tell him a story that captures his interest and sparks his imagination -- and helps him to see himself in a new and interesting role....
"During medieval times long ago, in enchanted kingdoms filled with tall, silver castle spires that glistened in the sunlight and deep dark forests filled with secret paths and enchanted creatures, a brave and courageous knight headed off to do battle with dragons could sometimes be a little bit scared. You can understand that, can't you? No matter how brave you are, who really wants to go off and fight a big, ugly, green, fire-breathing dragon?
"Can you guess what some of those knights did to remind themselves of the people they loved, the people they were fighting to protect? They sometimes borrowed a special piece of clothing and wore it into battle. Underneath all that silver armor, a knight might be wearing a pretty little scarf from his favorite princess, or even a handkerchief his mommy had sewn for him!
"When I see you wearing my pink slip, it makes me laugh a little bit, because it's so big on you, and doesn't fit you at all! It's made for grown up old ladies, like mommy, and not for little boys like you! Just see how big it is? Isn't that funny?
"But it also makes me think about how knights used to do the very same thing. You're my handsome little knight, did you know that? I see a lot of knight-like qualities in you. You're brave and smart -- and I'll bet you could outsmart any old dragon, don't you? Would you like to hear some stories about brave and smart knights just like you and how they outsmarted dragons?"
Now you can redirect your child's to other things without making a big deal about the slip and without making him take it off. You've given him a legitimate reason to wear it while you tell him some stories about knights and dragons and battles -- totally gender-appropriate things that have entranced little boys since the beginning of time. Take him to the library next, and choose some knight/wizard books to reinforce this theme. I'll bet you'll capture his imagination and interest faster than you can get him out of your pink slip.
Then work together to weave his love of dress-up into the imaginary world of adventure he is experiencing through these books and stories.
Take one of your favorite articles of clothing that he really enjoys wearing and make a wizard's cape for him that looks similar to this dress.
Whatever you do, don't try to impose upon him a dark gray and boring wizard's cape if his eyes are big for your red seqined cocktail dress. It's just not the same. The same things that attracted you to that dress are the things that are attracting him!
I have a children's party business, The Party Fairy, where we provide storytelling costumed characters and provide dress-up, and over the years I have seen so many little boys who are attending a princess party for a sister or a friend want their own nails polished, and fairy glitter dusted on their faces, and to be able to wear wings or choose a beautiful princess dress to wear, too. Sometimes dads look absolutely horrified to see their 3- or 4- or 5-year-old little boys head for the makeup center instead of the knight costumes!
But amazingly enough, most of these little boys fall into the under-6 age category. Something happens to most little boys around the age of 6 or 7, even the ones who have previously loved dressing up and playing with a sister or a best friend. By age 6, most little boys have developed a disdain for all things pink and princess, and avoid things that sparkle the way little girls of the same age avoid worms and bugs. They walk into my store making faces and asserting their gender identity by making fun of the sister who is dancing on air to be surrounded by such fairytale glitter and glitz.
That's probably going to happen with your son, too, but in the meantime I really would pursue the dress-up trunk and stories of adventure. It's a subtle way to reinforce his gender identity and has the added bonus of fostering early reading skills, too!. It can be the doorway to what becomes a lifetime love of literature.
Good luck !