Oh, I'm so sorry it's meth. IIRC, meth is the hardest drug to quit. Physically and mentally it affects users so much.
So first of all, a big hug!
Second of all, I think the PP suggestions are correct in that there's not much you can do until he wants to quit. That's true for most addictions, but I think particularly with meth. Remind him that you love him and that you will support him in quitting as much as you can, but don't enable him in any way. Remember that anything he does while he's using is not personal to you, and that he can't help it.
Early intervention specialist and parent consultant since 2002.