Blue never gaits when he's at liberty. If he's in a hurry in the pasture he paces or canters. His stride at these times is very lengthy and easy.
In the round pen I can get him into a gait but he seems to tire very quickly and starts to stumble. It's only at the gait that he short steps and stumbles. That's why I was wondering if he was just never trained to gait properly and didn't develop the correct muscles. I know that gaiting seems to come more easily to some TWH than others.
But I think I will save up some money and start with x-rays and a chiropractor before I put money into a trainer. It could be that he's hurt, untrained, or just doesn't have the conformation to gait well. Seems like it's a mystery I need to solve!
You description (tires very quickly in the round pen) suggests a horse that is weak, out of shape, and doesn't know how to carry itself properly at the gait.
If the conformation of the horse is at least good and there are no clear signs of injury or mal-formation then my first approach would be to improve strength and conditioning.
I'd start with lots of time dog walking. This can be done in the arena or on the trail (the latter being less boring for the person). Start with walking on a loose rein for a couple of weeks, then begin work on a collected walk. The best bit for this kind of work is a snaffle with a mouthpiece comfortable for the horse. After a few weeks add hills. After 8 weeks or so you can begin to add short period at the gait (like maybe a couple of minutes, and maybe even less). Walking will build strength; periods at the gait (and particularly the canter) will build wind. Indeed, in the round pen you can just use the walk and canter and ignore the intermediate gait until more strength and wind are built.
The soft, intermediate gait takes more strength and conditioning that a trot. The only reliable way to improve the soft, intermediate gait is thoughtful work. This presumes, of course, no physiological reason(s) why not (good conformation, good husbandry, good tack selection, good riding habits, etc.).
Keeping a horse in shape for basic work requires work four to five days a week, for a minimum of about 30 min. per session. No pill, device, adjustment, treatment, or "program" will do.
Put another way, there is just no substitute for "wetting the blanket."