This won’t help you since you said he can’t do 24/7 turnout but I thought I would throw it out as food for thought. We have gotten a surprising number of retirees who come to us due to GI issues. Their owners do the dance, as you have been doing, for years, and finally when the horse reaches late teens to maybe 20 they decide they’ve done it long enough and throw in the towel and retire. After a transition period these horses live out on good grass for much of the year 24/7, and mixed grass hay the rest of the year (along with grain and any needed meds/supps). A handful of our residents also get alfalfa pellets mixed in with their feed as well.
Once they hit 24/7 turnout the loose stools and symptoms of minor gas colics usually disappear within a couple of weeks. I will say that this specific type of horse tends to lose weight during the first 60-90 days almost like clockwork. I don’t know why. Then they gain it all back during the next 60-90 days. The GI stuff is gone forever. The maintenance doses of GG and/or ranitidine, periodic dosing with metronidazole, the bazillion GI and ulcer type supplements - they all become a thing of the past. The key appears to be two things. The first is being out with a group. The second is being out in a large area where the horses get a lot of low impact exercise through walking - our retirees get on average 7 miles per day (we’ve put pedometers on them).
We have seen 100% of GI issues, ulcer issues, irritable bowel type symptoms, etc. go away. Interestingly, when one of the two things listed above is removed (the group turnout or the 7 miles per day from regular walking around a large pasture), the symptoms return fast. The takeaway that my large animal nutritionist hubby draws from this is that some horses HAVE to have that continual movement for their GI system to work properly. The vast majority of horses can do fine with the modern lifestyle of more limited movement that comes during smaller windows of time, but some horses simply have to have the movement.
It’s tough owning one of the “special” ones, sometimes you feel like a hamster on a wheel trying to manage the various issues. Then of course there’s your bank account that has to be managed as well.