Just be honest with him, if you need something to lead into that particular topic of discussion just go "I have to ask, are you hitting on me?" and regardless of what he says, you can go with the "well, I appreciate that but I'm straight, I'm sorry" or the "haha, oh dude I just thought you did, and I wouldn't have known what to say cos I'm straight" kind of joke.
Either way, it gives him an opportunity to hide or be honest about whether he is hitting on you or not, as well as giving you an opportunity to make your own sexuality known.
Either way, it gives him an opportunity to hide or be honest about whether he is hitting on you or not, as well as giving you an opportunity to make your own sexuality known.