My little boy has stomach pain, done countless tests, spent a fortune, I’m at a loss, need advice?

My little boy will be 2 in January and hes always had huge problems with stomach pain and wind. I have taken him to the doctors so many times and had countless tests done. Hes had a general blood test, allergy tests, poo samples taken and physical examinations of his tummy and an abdominal scan. All results are normal. Every night I sit for 3 hours cradling my little boy because he is in such serious pain he has now started to shout tummy when the pain is coming. I have kept a food diary for the last two months and not been able to pin point a certain food that upsets his tummy. He is on Movicol for constipation and this is working fine and he poos regularly. It might be useful to mention that when the pain comes on he also coughs and clutches his throat, his voice is horse all the time and everyone comments on how deep his voice is for a baby. I have trawled the internet for the last 6 months looking for answers. I have spent a small fortune on various medicines to help with wind and tried natural remedies such as peppermint. Im at a loss now I’m on the edge he screams for hours and hours and my GP has told me I cannot take him back again as they have completed all intial investigations and found nothing. Is it possible he is suffering with GERD? I’ve read about this online and his symptoms are so similar. Please can anyone give me any advice.