I think that a lot of men with low libido may have some sort of underlying issues. My husband has gone through times where his libido is lower than usual, and it usually correlates to some sort of work related stress.
I would think back to when you first were together. Was his libido always so low? Is there anything going on in his life that might affect his libido? Contrary to popular opinion, men aren't always "on." They need their their fires stocked and tended to just as much as we do. See if there is an underlying issue. Maybe there is more to it than meets the eye.