I'm so saddened that this is happening to you and your son.
Meth's illegal (which might give you some recourse (which I would be tempted to take especially if it involved stopping the supplier who is no doubt ruining many lives).
Meth eliminates the ability to reason. I have a friend who stopped caring about her children and her husband in pursuit of this drug. Incarceration helped her with the addiction. She had extensive rehab and after many years reestablished friendship with a son. She is now employed in the medical field and is one of the happiest individuals.
Yours is a really tough situation.
Wishing you and your son all the luck that it takes.