He talks about the way you look, offers to buy you things. Insists that you're smart, confident, etc.
I guess he complements you a lot. Straight guys and/or guys that aren't interested generally wouldn't do that. He might also often look you straight in the eye, smile a lot, stand closer to you than than most other people and laugh at nearly everything you say. If you're at a bar, he'll ask if he can buy you a drink.