
Welcome Westie57,
My goodness you have your hands full! And I can totally understand why your ex is burned out. You are asking a question that is heard far and wide around this page. Although I must lovingly disagree with you on the boundaries. those can be started at any time and in any place. The hard part is that it starts with you since your son is now staying with you. I can see that you have already started one: not providing him with spending money. that's a great start.
I'll share a snip it from my many dealings with my D. She has always lived with me, at one point she was into meth and running the streets all night, bringing home strangers to my house. I told her one day I had the locks changed and wasn't giving her a key. If she wanted to run the streets at night she would no longer have access to the house unless I was there. I do hope that your day time limit is working with him.
Any way I'm sorry to say that I don't have an answer for you on the motivation question. I am rooting for you, maybe you could plant a seed with your ex, tell him that seeking counseling will help him with ways of managing your son; and help him find ways of starting self care. And by the way... .what does your self care look like? You work full time, so do I; I know that self care doesn't come so easy when you work full time and the BPD child lives with you.