Haven’t had time to read all replies so not sure if it’s already been suggested, but my son was EXACTLY the same. Turned out to be a very tight posterior tongue tie.
Pointless going to your GP/HV about this, they are bloody useless with infant feeding issues. You need an IBCLC or Infant feeding nurse to properly diagnose.
Not sure where you are in the Country, but i’d HIGHLY recommend Milk Matters in Huddersfield. If that’s not possible then Google tongue tie clinics local to you. I would say he definitely needs checking out by someone who knows what they are doing in that field.
CMPA symptoms are very similar to TT symptoms. My son was diagnosed with CMPA but as soon as his TT was cut, he could magically handle formula. Of course, he was never CMPA.
Just a word of warning that once the tie is cut, it often tries to stick back together, so a fair few sessions of cranial osteopathy are needed and it massively helps.
Best thing I ever done getting my son’s sorted. He is now absolutely thriving.
Best of luck
I know how tough this can be, but you’ll come out the other side. X