Has anyone ever had a problem with their not wanting to up trails that go up hill , (steep hills) I am not sure what his issue is. He will be just fine , but when that trail starts up a steep hill , he will not go . You can lead him up just fine , but in the saddle he will not go. I am sure it is a fear issue but I can't seem to get him past it.

Sounds like pain-most likely hocks.
If your with someone else get them to lead and watch from behind, hind leg should move straight think pendulum on clock, not twisting,squatting,shivering, etc...

Or
Rider imbalance
Poor saddle fit
Snug breastplate

Multiple things could be happening
What about same hill going down-will he stay straight ?

how often do you ride this horse? if the only time he gets ridden is on a once-monthly 8 hr trail ride, it could be a conditioning problem. Please rule out a phyiscal problem before assuming it's behavioral. It could very well be he is afaraid but there has to be a reason. After-all fear is a response to a stimuli, not a stimulus itself.

Could be a pain issue, I am thinking hocks or stifles.