S.H.
in addition to the other tips... every time you change his diaper... I would wash/rinse his bottoms under some warm water... then dry him COMPLETELY, then apply the diaper cream. You want his bottom to be completely dry, otherwise you will be trapping moisture "under" the diaper cream and this will make it worse.
We did this for our kids and it helps a great deal.
You might also try and change diaper wipes, or just use warm water in a spray bottle to clean his bottom at each diaper change... and use a soft paper towel to dry him off....just dabbing, NOT rubbing vigorously. This was suggested at the hospital where I had both my children... this is what they did at each diaper change too.
Also make sure that his diaper "rash" is not a yeast infection or something... if other ointments don't work, then it might be. But ask your Doctor.
Oh, also sometimes some babies get a bad diaper rash when they are teething...
good luck,
Susan