Hi, m daughter was the same, except we were refferred to paediatricians due to her lack of growth and fussy eating.
Basically this is what they told me.
Give high fat foods, which i kno sounds crazy but theysaid at her age it will NOT affect her heart health as she grows up as it will not become a normal diet for her throughout her life.
Litterally pure butter on sandwiches, add butter to mash, if you can add butter to it then do it. Burgers, chips, sausages (all fried) along with healthy snacks of course. Try the high fat nuts if he will eat them.
It ispurely to get excess calories into him that he wont burn off, honestly it wont harm him to eat these foods, i kno it does sound awful all fatty foods but that will put the weight on him.
Now my daughter is 8 years old. To keep the weight and height within normal range i had to do this till she went school, and then she had cooked school meals and cooked evening meals. Which she still has now.
So cook a meal at lunch and at tea for him, even if it is different to the rest of the family.
Cheese is really good, high fat,whole milk, chocolate. and see how you go.
How tall is he? and exactly what does he weigh, i will compare his chart on my daughters and see how similar they are. If he is less than her or the same then i would seriously push for a paed referal, as she was pretty little.