one way to do that is to show him the big stick. we once had a married client where we had to inform his girl friend that the relationship was over. we made it clear that she would have legal troubles if she made noise. of course it helped that we gave her financial motivation.
the point is that he should understand that there will be consequences. do not actually sue, but use the threat of it. on the other hand, there is no guarantee that nothing will happen, but if you actually sue then everything will be on record.
it will be easier if you are able to accept and plan for the possibility that he will do his worst when you leave him. otherwise, you will be paralyzed by your fear.