You will need to remain calm and keep your discussion on point, i.e., short and sweet. He's not going to be able to focus on lengthy discussion and will tune you out. If you get emotional, he will tune you out. Be sure to look him in the eye when you are speaking to him.
If you are going to set boundaries, remember not to come off sounding like his mother and use the "I" word and not "you" and don't point finger of wrong doing to him. Again, that comes off sounding like mom or an authority figure calling him out on his flaws.
You can do this!
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
