That sounds like a tough situation, I'm sorry you're going through it... if he's on meth then he will probably continue to drain your money. Meth is one of the most addictive drugs you can do. It floods your brain with an unbelievable amount of dopamine (much, much more than cocaine even). Once you're hooked, it's the kind of drug that will make you betray everything you (used to) care about just for another hit.

I think someone addicted to meth (or other hard drugs like cocaine, crack, heroin, etc) cannot help but exploit those around them - they will reach a point where the drug is basically the only thing that is important to them.