You want your ex to support your plan? You don't have a plan. You have a child who is now an adult and who has been in trouble for years. When he was a minor, you never got him the professional help he needed. You spent 3 years "trying to hold him accountable" - what does that even mean? You lectured maybe? You brought him back home for 2 months but you weren't able to do much of anything except nag him, but it didn't make a difference. He finished school, but so what? What's the benefit? What's the point of him going to trade school? Even if he finished, which is doubtful, and even if he got a job, which is doubtful, what makes you think he wouldn't be fired by the 3rd day for not showing up on time?
Your ex is a huge problem, I agree. But you are not effective either. You want to see his finishing school as an accomplishment, but it's not much of one. If you were really tough on him, you'd have enrolled him in a counseling and rehab program, inpatient and involuntary if necessary. Instead, you tried to do it alone, and when it didn't work, you sent him to his father's. Through it all, you paid for his cell phone and whatever else.
"He somehow has money"?? He's dealing drugs, dear. Either that, or he's been stealing your valuables and pawning/selling them. Or both.
So sit him down, as suggested below. Tell him he gets clean and gets into school or he's out, and he has 1 week. Before you do that, put anything of value in a safety deposit box or with a trusted friend who would not let your son in the door. Then take his house keys, cut off anything you are paying for (phone, clothes, food, insurance, everything) and tell him that's the deal. Inform his father, but expect nothing in return.
Get into a support group for codependency and enabling. Your son has to hit rock bottom before he will change, and I fear you need to as well.
I'm sorry - it gives me no pleasure to say this to you. I have a stepdaughter who lived like this, and a friend whose son did the same thing. You have yet to admit that you are powerless to do anything about this boy because you failed to do it when you were his legal guardian. Now it's out of your hands. All you can control and change is yourself.