Gosh so much of this sounds like my 5 year old i had to respond! Similar story generally fine b4 school but we had some behaviour issues that we'd asked for help for from age 2 years but was always told we were doing everything correctly and it was just his age - treated very much like a neurotic mother. His eating had gone from eating everything to eating nothing by time he was 22 months.
He was diagnosed lactose intolerant at around 3 years. (had been on soy formula age 6-12 months)
In the last 6 months an iron deficiency was diagnosed, he responded well to treatment but deteriorated again when we stopped treatment so there is some sort of absorbancy problem.
He has just started school and behaviour problems have started in playground, being aggressive towards other children, concentration problems, fidgetty, can't choose what task he wants to do, gets very tense and angry over things he shouldn't be getting so angry over. He's a bright wee boy (school's words) and although doing very well with his work school feel he should be achieving more based on their initial assessments. i would say in last 2 months he's gradually deteriorated, and he's struggling making friends. We saw our paediatrician this week who has referred us to Child and Adolescent Mental Health services to see if he is on Autistic Continuum (ie Aspergers). I do see alot of his behaviours can fit this diagnosis, but language wise he is very good and socially he's never had a problem until the last couple of months which is weird. However he has been eating more dairy since starting school, sore tummies have increased again and twice in 2 months he's been terribly sick with sore tummies. Now the sickness could just be bugs as loads going round school, but it's hard to then be sure whether its that or a build up of the dairy (only a small amount of cheese and petit filous) and if it's the dairy, making him feel unwell then it makes everything else deteriorate.
I'm all for labelling if it's done correctly as a child can then be helped, but i would hate for him to be diagnosed due to a dietry effect.
I hope you get to the bottom of the problem. i feel I'm starting to move forward and we can get the right help for him now.