You cannot save him. I think you really need to take a hard look at the person he is now, ignore who he was, and go forward with that assessment.
Fact, he is now doing drugs and actively looking for more potent drugs.
Fact, he's cheated on you.
Fact, he - at the very least - has impaired judgement, at the very worst he is going to end up OD-ing or involved in a drug bust.
Fact - you need to protect your kid and your finances.
You need to file for at least a legal separation. If he is doing drugs in your home or in his car, he can risk losing them should he get caught. They can likely impound and confiscate either or both. Then there are fines, court costs, etc. that you may also be held liable for without a legal separation.
You need to require a drug test for unsupervised visitation or talk to an attorney about this. You have to protect your son and your finances.
[This message edited by Mousse242 at 11:19 AM, September 7th (Saturday)]