Just to add another story.
DS1 was like this. When he was 6yo, I got to the root cause of it which was an undiagnosed posterior tongue tie. He was swallowing a lot of air when he chewed and swallowed because his tongue was unable to lift fully to complete the "correct" swallowing action. The air was coming back up and bringing stomach contents with it, so although technically not reflux, it had the same effect. He complained of a burning stomach and chest as well as being sick in his mouth a lot. Omeprazole helped a bit, but not very much.
He had an endoscopy which ruled out anything sinister, but did show irritation and some crystals in his oesophagus, so it definitely wasn't nothing.
We had his tongue tie released when he was around 6.5yo and the "reflux" stopped overnight, I kid you not. Within a fortnight, he had loads of energy and he's never complained of it since - he's 11 yo now.
He also had a rake of other things going on which were also related to his tongue tie and orofacial structure. He had a very high palate which restricted the space for his eustacian tubes and his nostrils. He had ENT problems, slept terribly from birth, snored badly, bouts of apnoea and mouth breathing to name a few of the difficulties and by age 7yo, even my untrained eye could see he was in for a load of orthodontic problems too.
You've only asked about the stomach pains, so I won't write an essay on how I dealt with the other problems, but if any of them are also familiar, I'm happy to expand.
Unfortunately, I was fobbed off by HCPs all over the place when I told them it was all related to his tongue tie. Mainstream HCPs are woefully underinformed about it. They spout "might affect speech and licking ice creams" but there is much, much, much more to it than that. I had to dig and dig until I found the information and help that I needed.