I have a temp loop ileostomy and waiting for my second surgery to rid me of the bag. I've been told that I will experience the problems that your son is having, and that cream and wet wipes should be used after each toilet trip. They did say that the GP can prescribe barrier creams for this problem so maybe its worth a trip to the GP. Although my back passage is not currently 'in use' I still have alot of discharge and it makes my bottom sooo painful unless I, like your son, use baby wipes and sudocrem. It sounds like you need something a lot stronger so maybe see your GP :)
I'm relatively new to all this so not of much use, but there are alot of experienced people on here who I'm sure can offer something advice. 