I just thought I would mention a little something here about ulcers. A few weeks ago I went to a seminar put on by an equine nutritionist (from Purina)about making hay stretch during our horrible drought and lack of hay.
She mentioned a new study where they were able to put a camera into a horse's stomach during workouts. They found that horse's stomachs collapse during exertion and more so the harder/faster they work. So when a horse is galloping it is super likely, with a very small stomach volume avalible, a horse will get ulcers from the stomach acid being pushed up into the esphogas. It was reccommended that horses never work on an empty stomach to prevent ulcers. It was also reccommended that horses were fed maybe 30 mins before a workout but, not much more, so as to avoid ulcers. She also mentioned feeding alfalfa to prevent and treat ulcers. My vet/chiro said that alfalfa acts like a tums because of the extra calcium in it. If your horse leaves his concentrates (ie grains) for hay, he is trying to self medicate because he has ulcers and it feels better to stop eating grain and eat alfalfa, or grass hay if he has no alfalfa. It was reccommended to me by my vet to feed a small amount of alfalfa with their grain so they can help themselves heal/prevent their ulcers. Actually my vet said to take the horse off of grain for awhile if ulcers were suspected. I have had great success with alfalfa added to the diet to get my slow grain eaters to eat more normally. For example, it worked with a horse I had in for some socializing. She would not eat grain very fast and would beg for hay even with a full bucket of grain. I mean, to me it is odd for a horse to leave grain for coastal!
I took her off grain and gave her alf for awhile instead. Then I added grain back after two weeks, still feeding her alfalfa. When I sent her home a few months later, she no longer ate slowly. Vet said he was positive she had ulcers. My friend is anti-alfalfa, so she has not had any since she returned home 2 years ago and still eats her grain quickly (at a comprable speed to her other minis). So if I was told my horse had ulcers, I would feed him before I worked him and I would also make sure he had alfalfa...WITHOUT grain (if it were me) at least for a little while.
The other thing you can do is give him a vitamin b shot. It is amazing how being low on b vitamins can make you sluggish and "off." I get a vitamin b shot about once a week!
It is hard to overdose on b vitamins too. My vet told me 2 cc for my 300lb minis. Did your recent vet check include any bloodwork? This won't help you to know about b vitamins (stored in cells, not in blood) but you can tell a lot about organ function by a good bloodwork up.
One last thing, not that it is not ill-fitting tack or soreness, but assuming or ruling those out... the more horses learn, the more they have in their arsenal to use against you!
It is possible, if nothing else comes out being the problem, that he is doing what any of us would do...anything we can to get out of working harder than necessary! If left without supervision or any sort of guidance at work, eventually most people would begin to do the bare minimum. He may have just figured out how to get you to let him be lazy! Now he is protesting having to work harder than he would like! OR he could be confused if he was trained by someone other than you. At first he was strong in his training and understanding. After awhile, he might be confused without a refresher. I second the suggestion of taking some lessons with a good driving trainer. If they tweek one tiny thing and it makes you and your horse happier, it is worth every penny!
Ok, Ok, I have babbled on long enough...probably no one is reading anymore! LOL