Ulcers can be a serious problem if not treated appropriately. However, as you've said, treatment with Gastrogard is very expensive (because it works & there is still a patent on the product). However, you can try using Ranitidine (Zantac) which can be found very inexpensively as a generic brand - your vet may be able to help find it. It is more labor intensive, but worth it if you don't want to incur the cost of Gastrogard. Personally, I would not recommend the generic omeprazole (which is similar to Gastrogard) as it has been proven (in research papers) to be COMPLETELY ineffective - ie: waste of your money. Other things which help to decrease acid in the stomach are keeping constant grazing/eating activity (saliva has bicarbonate buffers) and decreasing the stress level (if he hates being near his neighbor, move him; if he likes being outside, arrange to keep him out more; etc).
A gastroscopy (looking in the stomach to check for ulcers with a small camera) is the only way to check for sure if the ulcers are there & I always recommend that if you are thinking of paying money for Gastrogard because if you find out there are no stomach ulcers, then you have saved yourself $1000