My old horse had ulcers, I gave her omeprazul once a day and then while I would groom her I would give her a few tums to help settle her stomach and create a barrier. Alot of people from my barn give their horses Tums for a wuick relief before we went to jump
You can get charcoal activated suplaments .. there good for them as far as I know
This is a bit of a long shot but feeding a small amount of Hard Feed before riding can help prevent them, this leaves a lining to stop the stomach acid splashing up and creating the ulcers. You can also change management, hay is better than haylage and making sure that they have feed most of the time can help. Also turning out for long periods of time is good for some horses stress relief because they can be caused by stress!
I would try the apple cider vinegar thing as previously mentioned you can check out JETeventing for more info on that, then I would try some Probios products. I use them with my horse to prevent ulcers. I hope this helps:)
Brewers yeast. powder or pills from a feed shop or from health shops. We give our mare 4 pills a day and that keeps the ulcers away.
Also if he/she suits it, a small handful of tiger oats a day help.
A popular youtuber is going through the same exact thing with her 8 year old ottb. I believe she is using a 4 or 3 day apple cider cleanse and then will feed probiotics for a couple weeks. I’m not 100% sure so if you wanna check out some of her recent videos her channel is “ JETeventing”. Hope this helps! Good luck and keep us updated