My children only had it as babies and toddlers. It took me being on ted diet plus omeprazol & zantac whilst feeding them plus them being on the same diet plus Meds
But, I also have hypermobility syndroms, & have ibs/constipation/acid reflux & whilst they are part of the poor muscle tone that goes with hypermobility the Ted did help somewhat also, I'm not intolerant in terms of tests & I am normally fine if I have some diary etc but too much & I need a free period for a while again. Too much salt/sugar/grease causes the same problem.
My kids are mostly fine, but one gets flare ups if she has milk, other dairy doesn't set it off but milk will. It's very tough with older kids because sensory issues limit what they will eat.
Sr is mostly in visable so could be happening &,not obvious, but does damage the oesiphagous. Zantac helps the acid go back down easier (&has two doses so can be given at the higher one) but omperazol cuts the acid so it doesn't come up so the throat gets to heal again. It does mean nutrients arn't absorbed in the same way though so anemia is a risk, but not keeping food down causes that anyways.
Has your consultant tried domperidone? My kids had that too as I have but from what I understand drs don't like to prescribe it now as it's risked to increased chance of heart conditions when older- but it works by helping the stomach empty out quicker, basicly helps the muscles do their job. & buca stem plus others can help with the sickness itself. All these can be taken together, & can allways be decreased when acid levels are under control.
The thing I found helped me way more than Meds or diet, is a supplament called bucastem, it has to be ordered from abroad &,is very woo for me as I don't touch that kind of stuff normally but it helped a friends reflux baby who wasn't coping & works v well for me. Magnesium is also useful for helping with muscle function to some extent, maybe not massively but I find some small difference.
Your son's consultant has a wide range of other Meds to try so good luck with that