How Hard Is It Not to Relapse on Meth, if You Have Been Clean for 180 Days?

My husband, secretly had been doing drugs, meth, for some years now. Was good at hiding it from me, then when busted, and went thru rehab,and now has been gone for months, can he possibly stay clean? He says he will never use again, but how can you trust that? What if he gets upset, mad, over stuff when he gets home? How do you ever trust again?