Why is he sneezing all of a sudden?

I'm visiting my Fiance, he lives in a 70's style trailer, and he has lived here for about two years now. For the past couple weeks, all he does when he gets inside the house is sneeze. His nose is runny, and it's driving us both crazy! This has never happened to him before the whole time he has lived here. So what could possibly be making him sneeze so much? Could some kind of mold or something be doing this? I need to do something about it because it's really not getting any better.