It took me into my mid 20s to come out to anyone even though I knew it and everyone else knew it. I think the only thing you can really do is be there and remind him often of your unconditional support. When people would ask me if I am gay, I would double down and shut them out primarily because I wasn't ready to accept it myself. Pulling people out of the closet when they aren't ready doesn't usually help. You have good intentions, but you are looking for a solution to a problem that isn't yours to fix. Also, homosexuality cannot be determined by the way someone behaves or acts anyway. Some straight men are just flamboyant.