Ok. If it's possible, I would suggest comparing the foods you have tried. My girl ended up being sensitive to both chicken and fish, and it was a huge bother to figure that out. The runs, throwing up, etc. Not fun.
I think it sounds like you are pretty price concious? If you were in Canada, I would recommend trying a small bag of Lifetime brand fish, and lifetime brand lamb. They are all life stages, and pretty inexpensive (not much more than the Purina or Iams), and lots of dogs do well on them. Try to find a food that has a single protein source, probably not grocery store Purina or Iams. Protein sensitivities are more common than grain sensitivities, and I like that Lifetime has no corn, wheat, or soy, which are the main dog grain senstitivies. Most dog foods have chicken, so I would start with a non chicken based food. There should be something similar where you live. Natural Balance had a lot of single source proteins. Even if you can't afford them long term, trying a few small bags to see what he does well on (and switching to pieces of meat/veggies as treats at the same time) should allow you to make more informed decisions about what is causing his stomach upset. I have heard good things about the Purina Proplan but not personally used it because of the grain content.
A lot of boutique pet stores allow you to return food if the dog doesn't do well on it, so if you are on a budget, that's a question I would ask them.
I am personally allergic to gluten, so lifetime/ProPlan isnt an option for me. Annie was on diamond Naturals beef as a puppy, and Taste of the Wild High Prairie puppy. I suspect the diamond naturals is a bit cheaper. She is now on Acana Classic Red, which costs me with tax $100 CAD for 2.5 months for one spoo, which isnt horrible.
Re grain free.... I am not entirely convinced by the excited frenzy around the dangers of grain free food. Definitely if you do feed grain free, look for a food with more meat (there are some that are almost entirely legumes) but the percentage of dogs fed grain free compared to the number of dogs who had DCM isnt that overwhelming to me. It's something to be aware of definitely, but I somewhat suspect the marketing people from the big manufacturers have taken and ran with it. There was, historically, the same issue with lamb and rice foods, but no one has suggested no dog should ever be fed lamb and rice dog food. If more conclusive evidence about particular formulas or proteins comes out, I will check again.
Hope you find something that works!!