Definitely try to get help for him. Tell him about your worries. People say that marijuana is a gateway drug, but I disagree. It could be that he wants to smoke weed to keep from relapsing with the meth. I can honestly say that I have been clean from meth for 2 years now, but I smoke weed everyday. I have never seen a problem with weed. I use it for anxiety attacks, and nausea. It keeps me calm, and helps me suppress my thoughts of other drugs. It could be that he feels that he is close to relapsing, and really wants to stay away from it. If you feel that weed is bad for you, and you dont want to be around it in fear of relapsing, then you need to get away. Tell him that you dont want to be a part of it, and that if he can't respect that, you will have to leave. Now as far as you falling back into the addiction, that is all up to you. Everybody that has used meth knows the burden that follows you after you stop. Whether you relapse is all up to you, you can stay clean girl. Even if it means standing up to your husband to keep yourself out of the line of fire of this evil drug.
Best of luck to you and your husband. There is nothing wrong with seeking help when you need it.