Any one else deal with this with their kids?

My 4.5 year old son is has always since he was 2 tell me his tummy hurts and i always thought it was him just being a kid and not wanting to eat.
And his old pediatrican had him on pediasure as he was always small and she thought it would help him gain weight and height. But after a year of drinking it he wouldnt ear anything at all and then would puke if he drank it. Took him to her and she said he needs to have it.
Well at 3 i tested a theory. I stopped pediasure and thre tummy aches went away but he lost tons of weight. So put him back on.
At 3.5 i took him completely off and tummy issues went away. He did lose weight but after 2 weeks he gained all of it back. But within a month he complained his tummy hurt. So i again took him back and a differnt dr said acid reflux and gave him meds. We had the meds and the tummy issues went away. And then i was told to give it to him as needed and so i did. He then complained like once a month.
So i saw a differnt dr. She listend to me. And then she suggested a peds gi dr and referrred us. So now we have our appt booked but while booking my son who is 4 was babysat by my mom. She called me and said his tummy was hurting and he was crying. She gave him his acid reflux meds and he went to bed and didnt complain. Then 1 week later he did the same thing with my dh. So i took hin back and they did an xray to see if he has chronic constaption. The xray came back negative.
Then i looked on the internet and checked chrons and celiac diesae. I had him tested for that and other things that can cause tummy issues. All tests came back negaitve.
So now on dec 6 he sees a specialst. And i filled out paper work and saw they wanted a list of his growth from birth. Which i had and tons of history.
Now this all on the gi doctor. If they cant find anything then its behavioral.
Have any of your kids gone through this. If so what was the diagnoses