Yes really. :p
First I'd suggest getting him seen by a quality farrier and chiropractor if you haven't already. They can really help your horse become more balanced and comfortable.
Second, try building up some muscle on him. Start small. Walk up and down hills at the walk and trot. Lunge him with a lunging system.
When trotting, see if you can feel what part of the trot is off. Uman used to have a wimpy butt, so I'd ask him to use it more by tucking his hip under, asking him to pick up his head, and gain more impulsion. I also did a variety of stretches before and after I rode to rid some of his stiffness.
Be sure that you warm him up well! Walk at least 5 times around the ring both directions, asking him for inside and outside bends, extended and collected walks, and possibly get him over some simple poles.
Does he bump/brush his legs together when riding? Do you put him in boots?















