I agree that since it is chronic, you need to take him to the doctor and let them diagnose (or not) the cause.
Regardless of the cause, however, the diarrhea IS caustic and will burn their tender skin. My SIL who is a nurse told me (when my 10 month old had roto- and was raw as could be and wouldn't even sit in a room temp. bath) that the best most effective thing to treat it is to:
1) use a soft cloth and plain water to clean him up at diaper changes = NO prepackaged wipes.
2) instead of applying ointments/cremes or powders, take a big fluffy cotton ball, soak it in Maalox, and apply it liberally to his bottom. Let it air dry (it is runny liquid so it takes a minute, but not too long).
3) repeat at each diaper change until healed.
It is the acid in the diarrhea that does the damage. And the maalox (or any other liquid antacid) will neutralize that acid upon contact, so that it can't burn the skin when he has the diarrhea. Once you try it, you will realize that it can be a little messy to apply (it is liquid after all) and he will think it is cold, so it isn't something you will want to continue once his bottom heals, but it really does work. Just buy a bottle solely for bottom treatment, lol. :)
hth