Not read all of your links but if I was you I would
Request a meeting with his teacher and senco. Write down everything and take it along. Tell them you want a educational phycologist called in. If they say no then tell them you are very concerned. That you want them to observe him and give you his attainment levels and a comparison with his peer. Can they call salt into school again etc
Go to your gp. With your list again. Tell them everything. Ask them who he needs to see but you insist he sees someone. A Dev pead again, ot again, cahms ( is that the right spelling?)
School and nhs never talk to anyone. School can act with knowledge. I think if your son is passive but struggling and they have no known concerns they may just assume he is a low achiever. In a class some has to be at top and someone down there at the bottom.
My son walks round with his bum hanging out. There is reason for that. Sounds like you have a valid concern to look for a reason