My 2 1/2 year old daughter has had eczema since she was about 6 months old. If your stepson's skin's already turned into blisters, it might already be infected. You need to take him to the doctor. That happened to my daughter once, and she was prescribed antibiotics and elidel. You will see it get better. It is okay to put elidel on top of the blisters.
Unfortunately, you can't get rid of it. It is an allergy. The only thing you can do really is help maintain it. It is the disablility of the skin to keep moisture in. Also, the more they scratch, the worse it gets.
In addition to what the doctor will prescribe, right after his bath, you can put elidel on his skin and then put a layer of eucerin cream on it. You need to do it immediately after. This way you're trapping more moisture in his skin. I do it with my daughter all the time and her eczema's under control. Also, when it is moisturized, he itches less.
In addition to this, I also give her this mangosteen juice called xango. It is a puree of the mangosteen fruit and it helps prevent skin disorders. Her skin has never been this clear. You can go to xangopost.Com if you are interested about this product.
You need to tell his mom the importance of taking care of his skin. Otherwise, it will just always be infected.