I can hear your frustration in your response. Having had 2 dogs with inflammatory bowel disease I completely understand it. It sounds like you have tried many of the things that we recommend.
I'm sorry, I re-read your write up and while I see that this is a long term problem I don't see the information that it has been happening since birth.
Zantac is ranitidine, and it is a H-2 receptor antagonist and mild prokinetic. I didn't see that he had been given that drug in your write-up either.
Pepcid (famotidine) is a different drug, but its actions are much like Zantac so if Zantac didn't help the Pepcid may not either.
Prilosec (omeprazole) is a little more effective then Prevacid in dogs, so it may be worth a try. If given before bed it will have its maximal effects in the morning.
Finally sucralfate is a pure coating agent. If that hasn't been tried it is absolutely worth trying.
In some cases long term ulcers progress into cancerous lesions, so while this may not be a new problem for him it may be changing into something more aggressive. That's the reason that I think endoscopy is best.
I also didn't see any mention of combination antibiotic therapy which can be helpful if his ulcer is related to a helicobacter infection.
I know he is being fed a low irritant diet. There are several on the market. My own dog did poorly on Hills i/d but very well on Purina Veterinary Diets EN. Some dogs need a hypoallergenic food, such as Royal Canin limited ingredients or Purina Veterinary Diets HA.
Please let me know if you wish to discuss this further or have more questions based upon what I wrote. Chronic illness are wearing on everyone and I've been through it with my pup so I'm more then willing to help if I can.