Daughter #2 was the variety that splattered the wall with every feeding. I simply refed her - talk about producing milk like Elsie the cow. The second feeding stayed down and she was a big, healthy girl. Her doctor and pediatrician never found a reason. I don't think it ended until she was six months old.
Daughter #3 was allergic to my milk. She didn't vomit, but had terrible diarrhea. At six weeks, she weighed the same as when she was born. The doctor put her on a formula with no animal fat. The animal fat was removed and replaced with a vegetable product - coconut oil, I think. She immediately did very well. Her skin cleared up and she gained weight nicely. This was 49 and 46 years ago.
A granddaughter had your son's trouble until she was put on a formula with no iron.
What to do? Find another doctor if yours isn't concerned. If the baby isn't thriving, you keep at it until he is. Going back to nursing and avoiding all milk sounds like a possibility.