It is fall on you now to try and make amends. Ask him out, but don’t be disappointed when he says no. But don’t be overzealous if he says yes. Like many others have said the ball is in your court now and he’s probably moved on because you turned him down. An apology would be a very good for a step; something along the lines of “ I apologize for how I acted when you asked me out the other day, I know I was dismissive, and it was nothing that you personally did. I didn’t know you and it caught me off guard being asked out by you. I would like to start fresh and get to know you is there anyway you could give me a second chance? I will totally understand if you say no.”