My ex showed up at my doorstep (Super long question! Sorry!)?

Wish him luck and point him toward the nearest military or veterans' resources in the area, but don't let him stay. Your past history with him alone argues against letting him gain a foothold into your new life, but it doesn't seem wise to ume that a guy who would just show up on your doorstep unannounced and expect everything to go back to the way it was will listen when you say he's only welcome for a limited amount of time. You've moved on with your life, and he needs to start putting one together without you. The sooner he understands that's the way things are and gets going on it, the better.