Sounds like a case of he can't trust himself. He is young and like all young horses, does not have that experience of what it feels like to gait with extra weight on him and a bit in his mouth. Try doing some good, hard, trail work at a faster pace. Allow him to trot if he won't gait, because he will soon find it is much less energy to gait than trot. Thats why the breed became so popular for long distance riding horses. Keep working on going up and down hills so he can find his center of balance with someone on him.
(and when I say this I know I will be verbally shot down instantly, but it is what I believe helps and it has helped my horses, so I'm just giving advice that you can choose whether or not to go along with, its okay if you don't)
I like to have a nice canter every once in a while with my rockies. It seems to stretch new muscles and gets them limber. Some people will tell you that this ruins them. Not true. They canter in the fields don't they? It might also let him burn some of that youngster steam.
Good luck my friend. And whatever happens, remember: Your on a Rocky :)
Regardless of whatever happens, you must always remember: Your on a Rocky Mountain Horse