This sounds a bit like a flea allergy, especially as you say that he's been losing lots of fur recently. If this started happening before the frontline was applied, your cat could be severely allergic to fleas. This can cause bald patches and welts. Samantha lost nearly half the fur on her back, especially round her tail, and I was treating her with old fashioned flea preparations, then the newer types like FL came on the market.
May be worth a vet visit.