Originally Posted by horse_luver4e
Have you tried lateral bending? Bend his nose around all the way to his side, then mount up. When he is calm and he gives to the pressure, release him.
Sorry, but I cannot stand this method! There are a few videos of it on Youtube and I think it is one of the most horrible ways you can deal with a horse! Sorry to rant, just my opinion!
I would get him checked over for pain, teeth/tack/back etc, then if all is well, go back to basics, do some groundwork with him, and maybe get the handler to hold him, but make him stand when you get on, don't ride off immediately, then when he does stand, give him lots of praise. Like kitten_val said, make sure you don't accidently kick him as you mount and try and sit down as quietly as you can :) Really hope this helps x x x