Sorry I thought this was your daughter.
I would expect your son’s pediatrician to at the very least be giving you some sense of the timeline for this issue to resolve (if that’s what they are thinking) - so at least you’d have some sort of end date for what you’re going through.
It sounds like the reason why he’s maintaining his weight and not being a failure to thrive case is the extreme attention you guys are paying to keeping him fed - which doesn’t sound sustainable for you or anyone to keep up.
The catch22 is that due to your care and devotion he’s staying as healthy as possible - thereby avoiding crisis - thereby not triggering the system to step in.
Easy for me to say but often hard to actually make happen - but if you can get him back in front of a pediatrician that could be helpful. If that’s available I would spend some time detailing a succinct summary of the current situation. Type it up well in advance and refine it over time with your wife. Be honest about how much effect this is having on your health, marriage, work, and mental health. If possible provide a copy to the pediatricians clinic staff in advance so they can read it before seeing you.
Separate from that also type up the questions you want answered (like when can we expect improvement? What’s the next step for us if there is no improvement or if there is worsening?) Bring that list to the appointment. Write down their answers.
If that’s not available you may consider doing the same steps (typing up your summary and also your questions) and going through the ER (but that sucks as an option as you know)