It sounds as if your son has problems. you are trying to get to the bottom of that.
Your ex sounds appalling, so I hope that being back to just the two of you might help with your son, or at least the amount of time and energy you have to check things out.
NO child steals for no good reason. Just because he appears not to care, doesn't mean there isn't something up.
Yes, you deserve your life. But even if it takes a couple of months, or even a couple of years, to get back an even keel with your son, it will be worth it, because next time you meet someone (who is not a despicable twat), you will be able to introduce him to a happier, calmer kid. If you accept that, rather than kicking against it, you'll come out of it better.
I would never have said this 5 years ago, but 2 young teens, and similar "just bad for the sake of it" and I've found different.
Btw, you say you're all miserable. Well, DS was already miserable, DEx deserves to be miserable and it's his own doing, you are naturally miserable because you've left a 7 year relationship. Don't expect everything to be ok right now!
Good luck