your son may be confused a bit himself...this just may be something he is dealing with and he can't just give you a yes/no answer.
i don't think it is at all unusual for young people now to experiment...in some environments (some colleges) it is very accepted so people feel free to explore. (i'm not saying people are 'choosing' their sexuality, just that they can be more open about finding out what their sexuality is)
it must be a shock to find out something significant about someone you are close to and thought you knew everything about. but it sounds like you will be ready to accept him and support him whatever happens. that's really wonderful and he's very lucky to have such supportive parents.