I suspect he's been messing around with drugs a lot longer than he has let on.
Addicts are not competent to parent. Absolutely nothing good can come of exposing your children to anyone in active addiction/early recovery.
Maybe down the road when he has 1-2 years clean time, is employed, paying child support, has his own car and is paying for insurance...stuff that adults do.....he will have earned his way back into his children's lives.
Absolutely no reason for you to feel any shame about his choices.
You did not cause this.
You canot control this.
You cannot cure this.
Ask your family for help.