My son had the same problem, but when we switched to soy he stopped eating after 1-2 ounces. It turned out he hated the taste of soy, so he refused to eat it! Dr. put him on Nutramigen, but he was still spitting up tons and tons and crying in pain with every feeding, even between feedings. He prescribed zantac and it worked wonders. He is now 6 months old and back on regular formula (no soy, no nutramigen) and he still spits up a little, but he is not in agony.
If you think your baby is in pain, talk to your doctor because he may need a prescription. If he does go on zantac, take a hint, add flavor when you fill the prescription!! It is pretty nasty without it!