How to ask him if he's gay...?

[Stephen]

If you don't want to come out to him, why should he want to come out to you? And at 16, I can understand entirely why you wouldn't want to. But you can't expect someone else to do what you won't do yourself. And that goes for just about anything.
About conversations: it is pointless to force a conversation. You have interests in common. Talk about them. When you're through with the topic, just sit quietly. Nothing wrong with being quiet for a while and enjoy the surroundings, watch others, etc.
Good luck.