Ulcers?? No Lyme??No — help!

Long post here:
I have a 10 yr old TB with a history of starvation/vitamin & mineral deficiency from 3-4yrs old. When I acquired him he was underweight and severely vit E and selenium deficient. He has always had skin/ flank sensitivity that has never gone away. Some days are worse than others. Recently he dropped some weight. He has also had some other strange behaviors such as: new spooky/ look-y behavior, “broncing” when being girthed, tripping behind, reluctance to move off the leg laterally, anxious when holding up hind legs/hind shoes being done for farrier to name a few.
Keep in mind these things do not happen everyday but have all happened more than once. He had x-rays taken of his neck, back, all 4 legs including feet, ankles, knees, hocks and stifles. Nothing of any concern seen. No kissing spine. Negative for Lyme, low positive for EPM but treatment did not change any signs so likely just had exposure at some point. Negative for gastric ulcers on scoping, all bloodwork CBC/chem/selenium/vitamin E/muscle enzymes all come back normal. Had previously tested very low for selenium (5 ) but is in the normal range now. He’s on a low sugar Whole Foods diet with 24/7 access to hay, multivitamin, selenium & vitamin e. He is worked 3-5days a week with a mix of groundwork, dressage work and hacks/trails

Considering the symptoms, the vets have hit the obvious plus some. All were convinced he had terrible ulcers but he was pristine. We understand he could still have hindgut ulcers but wondering if anyone has had any thoughts ?!

Our next step is bloodwork after exercise and see if there are any other changes to his levels indicating a potential PSSM/muscle issue

wondering if anyone had dealt with anything like this before?? I’m at a loss

I agree next logical step would be to look for RER or PSSM

Does he have any digestion issues? Any history of colic? I presume parasites have been ruled out? Have you done bloodwork and abdominal ultrasound to look for signs of colonic ulcers? colonic ulcers would affect the protein levels in his blood. Also it wouldn’t hurt to try the Albumin stool test?

What about his hocks, Saddle fit?

Best wishes!

Our next steps are going to be the PSSM and ultrasound for colonic ulcers. I have been using 3 different vets and all have been trying to put their heads together with this. Saddle fit is great and he goes quite well under saddle which is why the change is behavior is so strange.
he has no history of colic in the 5 years I’ve owned him and he has fecal tests done quarterly with working as needed based off the fecals so that shouldn’t be the issue.
hoping to have some type of answer soon.

i should also add that he gets better as the ride progresses, not worse which is also throwing the vets off