M.P.
How do you know he's losing weight? Do you weigh him on baby scales? I suggest that you find a pediatrician and then arrange to take him in to be weighed once a week. Losing weight is indication of a possible problem. By having the office do the weighing the pediatrician will take you more seriously.
Babies do sometimes spit up after eating, especially if they drink too much. Does the spit up look milky at all. If so, I suggest this could be normal.
You need to take him several times to the same pediatrician so that they can see his progress or lack of progress. Just one visit doesn't usually get a diagnosis and treatment.
And the ER is only for emergencies. They will not be able to treat a chronic condition. An emergency is when the condition is possibly life threatening.