Probably something you'll only be able to determine if he comes out of the closet or starts flirting with guys too much. There used to be a gay guy at my work. He was pretty nasty, ended up getting fired for it. (sexual harassment, more or less). That was way before I worked there though, only heard about it. I know a guy who's gay and while he is a rather interesting character, you'd never really know he's gay. He's not the type that is "in your face" about it. I've hanged out with him plenty of times and have no issues.
Personally while I know it's wrong according to my faith, it does not really bother me if people make that choice, just don't be all over my face about it. Sounds like this guy is just trying to be nice, would not really worry about it.