Thank you for the additional information .
Tough call here. I asked about the x rays to be sure they looked for a blockage. With the symptoms you have described this really needs to be ruled out.
Having diarrhea for a few days can make it painful to pass stool , it can also cause some straining due to increased urge to go with nothing to pass. It should not make him painful to the point that he does not want to move Gastritis can cause pain as well both in the abdomen and when passing stool but again not to this extent
You mention that you are leery of the antibiotic so I am not sure if you are giving it or not but if gastritis caused by bacterial infection it will not clear without the medication.
I know finances are tight but there may be some things we can do to help.
There are a few companies that offer credit cards just for vet care, these are low to no interest depending on how long you finance Click here
If your vets office is AAHA accredited( you will have to ask) then you can ask them to apply for a grant from AAHA for you
Most areas also have programs and low cost clinics if you can tell me where you are located I can see what is available for you