I understand the concern over a pain issue, Angel, but just because he's 7 doesn't mean anything as far as balance goes. I have a 10 year old that was handled roughly when she was started, and she's been checked out by vet, farrier & chiropractor, but is still unbalanced with a rider on at the canter. For her, it's a confidence issue. She is perfect at w/t but when she starts to canter, she takes about 3 strides, and panics, then tries to bolt, or crowhop, or buck...Doesn't matter how small the rider is, and quite frankly, I'm fine with her being strictly a w/t horse. She's awesome on the trail. It is different for them with a rider on, and some learn it easily, but a few have problems with it. It can be trained out of them either way. I guess what I was trying to get at was to give the trainer a shot to train it out of the horse, and if you're set on keeping it, and things haven't improved, have a vet check it out, but a decent trainer should be able to tell you if the horse has a pain issue or confidence issue WITHOUT having to make a vet bill unnecessarily.
Ask Often, Demand Nothing, and Reward Generously.