Be interested in something about him (it needs to be genuine interest -- if there's nothing about him that interests you, then why are you bothering?). Display that interest. Ask him questions (not nosy questions though).
If this draws him into conversation, then he was waiting for a green light. If he seems uninterested in talking to you, then move on
Much more eloquent than my answer.
I forgot to add: once you and he are comfortably talking, then you can mention that your weekend is free, and you're at a loss for what to do to spend your time. That's his gold-trimmed, hand-delivered invitation. He'll either suggest some mutual activity, or it's your signal to move on