It's a terrible situation for the kid but you're a kid yourself. Do you really want to start your life out with a meth addict?! Really, come on. You can't help him, you can't save him, you can't convince him to quit. The best would be to contact child services and let them know of the situation and then leave and cut off all contact with the guy.
Seriously, if you stay, you're throwing your future away.