These traits are far too little to go on. Especially for a 16 year old it could be any number of things that causes him to not talk about sex/girls - he could be self conscious about himself/his body/his chances with women, he might have a very low sex drive compared to others or be asexual, he might just be religious and reserved.
If you want to know for certain without asking, then you need to get creative. People are good at lying if they have time to think about it and prepare, so you're going to need to find ways to push him out of his comfort zone and see how he reacts in the moment. Getting him drunk is a great idea. Asking him about his religious beliefs and family life can give you clues. Literally finding a way to show him two guys kissing (maybe in a movie or something) and seeing how he reacts is obviously another one - if he is awkward about it he is more likely to be gay or asexual, if he has no problem with it then he is more likely to just be well-mannered and slightly reserved.
Another thing you can do is start saying lewd things around him and see if he cottons on. If he looks at straight **** then he will know what a **** is, he will understand the innuendo if you say 'she must have nice lips, and i aint talking about her smile LOL'. If he looks at gay **** he probably knows what bottoming/twinks/edging/prostate/perineum all are so if you make jokes using the lingo, you can see if he laughs and understands them.
I mean, it's morally 'dubious' to test someone like that rather than just asking them and letting them tell you if they want, but on the other hand who cares.
