When someone leaves their house without taking most of their belongings, just clothes and bare necessities, it's usually because they're sleeping over at another persons house; ergo, an affair. He's cheated on you once. The likelihood that he's cheating again is pretty high. You can't take him at his word.
You need to find out.
Best strategy in this case is to focus on yourself; GAL, DB like crazy, all the usual stuff you hear on these forums. Don't involve your MIL, that'll push him away further.
