Ol'Line Rebel

1st, you must distinguish between "vomiting" and "regurgitating". GERD/reflux problems do not cause vomiting. You CAN have regurgitation into the mouth, etc.

Regurgitation is an incidental slippage of below-mouth contents into and/or out of the mouth, because there is little control in the upper tract.

Vomiting is the getting-sick thing, where the STOMACH forcefully and deliberately expels its own contents. The tummy will indeed seem like a muscle rather than just sitting there. As a result, sometimes you feel like your gut has had a work-out after a session - exactly because it did!

In theory, GERD problems are the very definition of regurgitation - if the sphincter down there is just open, you have no control over the stomach contents moving by gravity, e.g. Extreme case, you'd have an open sphincter, and standing on your head, everything would just dump out because there is nothing "sealing" the stomach.

Probably another sign of vomiting is how much he emits and how repetitive it is. With true vomiting, basically you suffer several heaves in a row at least - the stomach is really working it. Regurgitation probably seems only accidental, that he just found he has a load to get rid of, and probably only comes out once instead of repeated heavings.

I don't know why your son would have to feel any kind of "pain" from GERD regurgitation. It just happens.