If he is really vomiting and not just spitting up ask them about pyloric stenosis. Some may say it has to be projectile vomiting - it does not b/c neither my son nor I did. Your son will need to have lost weight and strength also. I had this and my oldest son had this.
This is a condition where the opening from the stomach to the intestine has closed - it closes after birth. He may need a upper GI test - that is the only way to test for pyloric stenosis. My doctor had me change the formulas too and he kept throwing everything up. The only thing that stayed down was water. When I mentioned I had had pyloric stenosis when I was a baby, she immediately scheduled the GI test.
It does entail surgery but everything went just fine....I hope it is something else but I thought I would throw this out there b/c most doctors have no idea or even think about looking into that diagnosis...
If you have any further questions just send me a message.