Although I obviously can't be sure without seeing the rash, what you describe is consistent with eczema. This inborn skin sensitivity comes and goes, and varies from a few spots to occcasional widespread outbursts.
Regular use of steroid creams does not "make matters worse." On the other hand, they often don't help when the condition is extensive. Neither does Benadryl.
Your fiance needs proper diagnosis and effective treatment so he won't be miserable.
One of the useful things about us dermatologists is that we don't do many tests, because we don't have very many to do, and besides, we can often tell what we need to just by looking.
I therefore suggest that he consult a dermatologist who can:
1. examine him
2. perform a skin biopsy if that's needed, which it may not be
3. recommend therapy, whether topical or a short course of oral cortisone if that's approproate
4. there are now non-steroid creams effectiove fro this condition, and they may also be worth considering
Of course, this may be something other than eczema, but he'll need a visit to find out. From what you say is his degree of discomfort, it sounds like a worthwhile investment to find out.
Best.
Dr. Rockoff