Why are you leaving things up to him?

If he proposed to you and is now seeing his ex who has these problems, it sounds like you are dodging a bullet if you leave him and YOU make the decision. Don't waste your time imagining what is going on. Step away and move on and look for greener healther pastures.

Marriage doesn't change toxic, this is a red flag you need to listen to. Marriage can bring many more problems than it's worth and it's not the be all and end all.