If he's truly interested in you, it's not going to take much of a push on your part to get him to ask him out. It may not take any more than an e-mail or a text message (though I recommend a phone call) to let him know that you would be up for a date.
If you don't prefer the direct approach, then you could use the same rumor mill that you used to find out he's interested to pass him a message that you're interested back. I still recommend the direct approach, though - just call the guy. He'll ask you out.
Don't worry too much about the mixed signals you threw at him before. If he's interested in you despite anything you may have done in the past, then all signs point to a first date some time in the next couple of weeks.