We went to the vet today and she seems to think the problem is fleas. We haven't seen any fleas, but he has started spending a lot more time outside lately and she said that the discoloration on his hind quarters (as opposed to somewhere else on his body) makes her even more suspicious of fleas. She prescribed Frontline and said to come back if that doesn't make his skin return to normal. She said that even if that isn't the problem, she doesn't think it's anything serious and that the next thing to do would be to test for allergies.
Thanks for the feedback... hopefully this will work and he'll return to normal!