Your penis starts swelling and enlarging. Its called an erection.
What Happens When A Guy Gets Sexually Aroused?
If you're a guy, you will know it when it happens to you, there will be no question at all.
He gets a boner, and wants to stick it in a chick...or if hes gay a guy...
They go crazy, at least my ex did. He went all over, I mean ALL over. Take a chance.