There are some simple things you can try...on condition that: no illness, hoofs in shape and ballance!
1. ask him to gallop without a saddle on him (Please do this only if you're comfortable w/o a saddle!). This make sure it's not the saddle how scares him or make him uncomfortable. At first, try this during a Groundwork lesson!
2. ask him to gallop middle in the corner or in a small circle (on both sides). That makes it diffucult for him to lift his hindquarter. Make sure his head is not higher than his withers!
3. try to lift his shoulder a sec. earlier as you cue him to gallop. You can try this out from a stand, walk or the cowboy gear. Out of a stand still: set him back for a half step to get the right leg position, lift his schoulder and cue him softly. From the walk: check that the right frontleg is in position, lift the shoulder and cue, with your other leg in position direct on the girth. Out from the cowboy gear: put a bit more pressure to both legs, lift his shoulder and if he comes up slide on of your legs 3 or 4 inch backward followed by a soft touch.
4. check if you ask him with your leg to far back. Try to change your leg position each time only half a inch backward till you get the result you want.
5. try to use good spures to touch him with the wheel. I mean a real soft touch, 3 to 4inch behind you normal leg position.You can do this by lifting your heel with consistant pressure down in the stirrup.
Try only one thing a day. If it works, work on with his ground behavior. If he's healthy with hoofs in ballance and condition, he need some more lessons to trust you. If you have some more question or need more help, feel free to email me.