he sounds just like my dd. she has a wheat intolerance, it's not an allergy so doesn't show up on blood tests. the effect is so obvious though that even a previously sceptical barrage of docs were convinced. when we took wheat out of her diet she gained 2kgs in less than 2 months immedietly, started passing proper stools and had a complete personality change.
when she has wheat it damages her gut in such a way that other food isn't absorbed either so she stops growing, passes loose undigested stools and is in pain and cranky all the time. she also has very low iron levals.
from the age of 12 months until 22 months she gained no weight at all and at her smallest she weighed the same as my then 6 month old ds1
i'm glad your HV is good, our PHN [HV] was a godsend who helped get us a paeds referral and then when it was taking months for dd to be seen she and the dietician caused a fuss and had her bumped up the list to be seen.
dieticians are great too, they're far more aware than the docs about the host of food intolerances that might not be allergies but can be just as severe. doctors can be very black and white in the way they view thing.
at the end of the day you know there's something wrong so please don't let anyone make you doubt yourself... don't be afraid to be pushy and aggressive with them if needsbe to get him diagnosed and treated.
it might be no harm to test him out yourself at home... try a week with no wheat, one with no dairy, etc. the differance is immediate when the cause is removed from the diet so it should be obvious within 24 hours if you've found the cause... it might not help but it can't hurt can it?
my heart goes out to you, we were where you are 2 years agon and it's emotionally exhausting. i hope you find the cause soon and you ds starts to feel better.