i'll just cut to the chase.
let him go. Since FEB he's left to go get high THREE times, that you know of. He lied when he said he had been clean for 3 years. he impulsively and stupidly spent all his money. he borrowed money from your TEENAGER for pete's sake to go buy drugs.
there is nothing good or redeemable to be seen here. you will get used and abused and end up wondering what in the hell happened. he's an addict in active addiction. and you and your precious son will be collateral damage. you'll have to trust me on this.