You kind of answered your own question. You don't have to help him do something you don't want, and you sure don't have to pay for it! Let him stew on that in his tiny flat.
MH is right. It was his choice to move out, you didn't throw him out. Let him figure things out for himself. The only ultimatum I have ever given my H was that if he wanted to end our M, he would have to do it because it's not what I want and I wasn't about to do anything to end it.
All you need to do is whatever is needed to protect you and your son.
You can do this TTD .
Both 50 S14 M 16 yrs (his 3rd; my 1st)
ILYBINILWY - 24 Dec 2012 H moved out - 27 Jun 2013 Legally separated - 6 Sep 2013 Closing the door and changing the locks