Sorry you're going through these issues. While sex isn't the deciding factor in relationships, it does carry some big weight.

The first thing that came to mind when I read his age (41) was that perhaps his testosterone has declined. It's normal for levels to drop as we age, and it would have a definite effect on his libido.

It could just be all the stress he's currently going through. But you said that this has happened to him with all his other relationships as well. The problem could be as simple as him getting bored with all his partners over time. Or maybe he has a fear of commitment and 2 years is the amount of time it takes him to realize you're not leaving anywhere soon.

Since it's a recurring theme, I doubt there's anything you can do. He may need counseling of some sort.