Listen to your gut instincts. You know what he's been like before, and there's a chance he is still like this.
I sense you're holding some expectations, which is perfectly all right and human. It's okay to hope that he has resolved his drinking issues, and has re-identified with the wonderful parts of himself that first attracted you. But do be prepared for all possibilities: whether he's drinking, whether he's seeing someone, whether he's being honest, whether he's truly listening to you and respecting your well-being. Be prepared to handle your own reactions and know what your boundaries are.
We did not talk about the past, and also didn't mention if we were seeing other people.
This really sticks out to me - you're both letting each other know that the door is possibly open. Once again, that's all right. But use this reunion to figure exactly where he's at these days, and IF you are interested, whether it would truly work for YOU. Don't get swept up to quickly in sweet, affectionate behavior - especially if it distracts from answering any questions. The most important thing here is that whether he has succeeded in sobriety or not, you will not let this chance encounter have the power to throw your life off balance.