I was going to say, I remember reading a post of someone who used long term omeprazole and it had disastrous effects on their horse’s gut health! I agree, the stomach needs some acid to help digest food, and long term acid reduction would be sure to cause issues.
Try looking up the Facebook group Equine Ulcer Research https://www.facebook.com/equineulcerresearch. Dr. Sykes is a long term researcher on equine ulcers, and I remember one of his lectures stated that each horse responds to omeprazole differently, and some have about the same reaction to the 1/4 dose of “Ulcergard” as they do the full dose of “Gastrogard.” (which are the same product, just labeled for different dosing). So basically, some horses only receiving a 1/4 tube effects their proton pumps the same way a full dose would. I would not consider a long term 1/4 tube dose safe for long term overall gut health personally.
I would be more inclined to try a supplement that will help coat the stomach lining. My highly ulcer prone mare is on GutX likely for the rest of her life, but I believe it coats the stomach enough to make her comfortable and help reduce the recurrence of ulcers after full omeprazole treatment.
You may find with time that your horse settles and becomes less ulcer prone. You said you got him in January? It would make sense to me that a TB that likely recently moved barns would be at a high level of stress, and more likely to rebound with ulcers right after treatment if not immediately placed on a preventative. Try re-treating and then starting on a proven supplement. It has taken both of my OTTBs about a year to settle into their new situations.