Advice dating someone going through divorce
You saying he's mentally all over the place, that would be a red flag to me.
If you have children, then you also need to realize that it's in your best interest to try to keep a cordial relationship with your husband.
It takes time to get over a divorce, much less getting through one. I had been told he'd be divorced for over a year, when in fact he had just been separated a couple weeks. If I knew then what I know now, I would have told him to go away and call me in a year.
Lot's of things to work through that you don't want or need to be a part of. When I was going through my divorce my counselor said to wait to date a minimum for 6 months post divorce.
Believe me, he will likely react to the fact that you are dating by making your life hell during the divorce process.
He may seek revenge to compensate for the anger, hurt, and embarrassment that he feels you have caused him.