It's hard to say, not being able to see in person...but based on what you stated, I might venture a guess that he is lacking in self confidence. Carrying a rider while moving out creates a whole new balance issue for them to adjust to...and how you sit the saddle can make a difference too. It might just be a matter of helping him build his confidence in himself as well as you.
I know this sounds like a broken record, but how's his saddle fit? Alot of times, a horse will seem fine with a saddle in the walk and the trot, but when asked to move into a lope, where the horse reaches out more, that is when many ppl, myself included, learn more about saddle fit and how it functions during movement. Check out this site by Kellie Sharpe:
http://awarenesshorsemanship.com/sadlfit.htmand
http://awarenesshorsemanship.com/saddle.htm"The trapezius, the muscle that raises the shoulder and moves it back and forth, will likely be affected by poor saddle fit. The latissimus dorsi, the long muscle in the back, will almost certainly also be affected. Deeper muscles in the back, shoulder and loin may be affected as well.
Kellie Sharpe"
(taken from the above mentioned website link)
That is why I say the saddle could be a possibility..if he fears pain, it might be preventing him from advancing to the lope...
But if he's young, and new at this, I would still lean toward him just lacking the self confidence..or a combination of both.
Good luck
[ July 31, 2007, 05:54 PM: Message edited by: Mustang Blue ]