This man is married and therefore unavailable. He needs to sort out whether he wants a divorce, he then needs to follow through on it, then he needs time to heal and reflect. And only then is there a chance that you two could be in a functional relationship.

If you two start anything before that time, it's very likely to fail. Don't get involved with a married man. You can't be the reason he divorces, you can't be his fallback, you can't be his mistress, all of it ends in disaster. He needs to decide on divorce (or not) on his own. He needs work up the courage to tell his wife, he needs to get a lawyer, he needs to work on custody, he is facing a lot of awkwardness and it's NOT up to you to support him through that. He may not even do it. Don't be his "we can be together someday, I promise" person.