5 year old only weighing 16kg?

i have a 5 year old son who only weighs 16kg we've just had tested for coeliacs which they said he hasn't got.I wanted him refer-ed to a pa ed my dr didn't seem concerned.he has had problems regarding Gastric reflux as a baby were he was tested at age 2 for lactose intolerance told it was atopic by this point i put him on soya milk.well at christmas old symptoms returned for at least 3 weeks he would vomit every night and i bout of runs in morn fine during day,he has lactofree milk and doesn't eat milk chocolate,went back and forth to different Dr's who all said he needs referring to a pa ed,last dr he saw last week prescribed gaviscon guess what all symptoms now resolved and we ve stopped gaviscon on prescribed for a few days yet,still complains chocolate makes his mouth hot .now i will have to pay private to see someone am i being a stupid parent,he complained he was cold at his swimming lesson as there is nothing on him to keep him warm,im sure this would be cled as neglect if you saw him.he is a terrible eater and so picky,ive two other child who both eat fruit and veg,we did avoid alot when he was a baby as many fruits used to make him vomit.what to do.