The fact that he foresees infidelity is a red flag. He also seems to have a few stressors in his life, which may be leaving him discouraged in many aspects of his life.

A feeling of "attachment" can't be forced. To regain it, work has to be put into the relationship on both sides. If he's isolating himself and refusing to deal with issues, that's not going to work. Maybe he's slightly depressed.