I agree with your husband! My kids were expected to eat all their food from about 1 year old. Here's how you would work your child into this.
1. Give him a spoonful of everything on the table.
2. Insist that he sit there and eat it all before being excused
3. Give him more from other things on the table as he asks for them.
4. Do not give snacks, juice or milk in between meals - only water.
It might be a bit of a struggle at first, since he's been free to do as he wants for nearly 3 years, but its important to teach healthy eating habits early on! Sitting at the table to eat is important even for adults, it helps us keep track of how much we're eating and also that we're getting enough fruits, veggies, legumes, whole grains etc.... Its easy to fill up on the fun items and not the highly nutritional ones when you're up and down all the time and not finishing your plate. Also dinnertime is family time. The experience of eating and sharing together as a family is very important. Kids should learn to take part in this activity early on!
My kids usually eat breakfast like champs. (Usually several fruits, whole grain cereal/bread, serving of nuts, soymilk) Lunch pretty well (either leftovers from the night before, or PBJ), and fight supper (if its something they don't like). My kids stay home, so sometimes (since they eat lunch much better then supper) I will make the main meal at lunch time and save some for hubby at supper. Then I have a better time getting them to eat supper. It just depends on how much they like/dislike the food. Then what I do for supper is:
1. A serving of fruit and whole grain crackers, toast or popcorn
2. Simple foods, fruit and or vegetables (steamed, no extra sauces, onions/garlic, etc... - i.e. boring for adults)
You'll learn what he likes and dislikes quickly. However don't let his dislikes rule what you give him, just give him a small amount instead of a large portion. Teach him that you eat your veggies because they are nutritious and not because you like them. Food is fuel. :)
I know this is difficult, it takes time and patience. But its really really important to teach him this now!!! You are lucky, your husband thinks its important. Have HIM be the one to teach your son, this will take some of the stress off of you! :)
Best wishes!