Well he's clearly got some problems hasn't he.

This apparently 'subservient' relationship with his ex is clearly very unhealthy but for whatever reason he feels he needs it.
Quite why is anyone's guess since there doesn't seem there's any advantage in it for him personally.

And while he's seemingly hung up on him that leaves little room for you......not in a healthy way anyway.
It sounds like he uses you just to get comfort from the truth he knows is true about this ex.
But you can't find any happiness in this situ.

Something needs to change and i think you're going to have to confront him on why he does what he does.

Lets say for the sake of argument, that he is not over his ex. Why does he have to be? When people play the "over your ex" card they are essentially saying that

1) they erase all emotions from that relationship away
2) that detachment is an easy thing or that attachment is fleeting with little value at all.

Maybe he is attached to her and maybe he is the kinda guy that forms deep bonds with women that don't break or weaken except over long periods of time or never at all. That isn't so bad. What would be bad is a guy who can let go of all emotional attachment to an ex on a whim which implies that attachment was at best superficial and at worst never actually existed at all.

Either way hope you two work through this together