@babyur4me do you know what medication the doctors prescribed for him?

I also use Google Scholar (https://scholar.google.com/) when searching for new treatments or trials. I did a search just now and found an article from the Journal of Dermatological Science, Volume 92, Issue 1, October 2018, Pages 38-44 which may provide some new information:

The effect of levamisole in the treatment of recalcitrant recurrent erythema multiforme major: An observational study
-- https://www.sciencedirect.com/science/article/pii/S0923181118303025