Can you encourage him to see a GP? It sounds like he could be possible suffering mental health issues, the bugs crawling isn’t a side effect of coke as far as I’m aware.
He needs to want to change though, you can’t make that decision for him as much as you want to try. I’m addicted to codeine and trying to taper off, as much as I love my girlfriend, her saying that she wanted me to stop taking had no bearing until I wanted to.
The most you can do is not enable any of his behaviour, if he asks for money ensure that it’s being spent on food etc, don’t give him money as it will most likely go on drugs.
Good luck, I hope he gets the help he needs