I will share with you the wonderful advice that I received from MY own Step-mom when I became a step-mom.
"You have chosen to be in a relationship with a man who has children. They are not obligated to love you, but because of your choice, you need to love them. Your main job is to facilatate a loving relationship between your "spouse" and his children. Do not take any rejection of you personally, kids are very loyal to their birth mom, just be open and accepting."
I am not saying that it is easy, but I pick up some things in your note: you say that the birth mom "claims" that the child has sensitive skin. Why would you doubt her? (maybe the reason you don't see any rash is because mom uses stuff kid isn't allergic too). More importantly, why put any negitive energy at all? Just use the same laundry detergent she does and let it go.....besides, asking her, "what is the best laundry soap for Jr.?" shows respect to her and her child.
No, it isn't easy, and sometimes as the step parent, you get the blame, but trust me, it is so worth it. I am loved by my step-kids in a special way and I love them too. I did not try to suplaint their mom, just to supliament!
If you do not feel comfortable speaking with his ex, write her a nice note and ask her, "what kinds of foods does Jr. prefer? Does he have any fears we should know about? (I wish I had asked this one before the first time the kids spent the night and 4 year old woke up screaming!) Ask the 5 year old if there are special toys he would like to leave at Daddy's? Take him with you when you shop for sheets and comforters and let him pick them out - anything to make the child feel that your home is his home too.
I could go on, but I think you get my drift......Bless you and Best of Luck!