9.5 year old underweight, which PD to see?

My son is 27kg and about 1.39m. He’s skinny, doesn’t really want to eat and eats terribly slowly. Anyone get this with their child? I’ve been to Dr Vera at Kinderclinic, she prescribed supplements, and said he’s a little skinny but still ok. But I find him pale and not much stamina when playing Soccer. Is it common that children at this age are skinny?
Good to see that this has been taken on by Wetherspoons, and is about to reopen as The Boot again. I wonder if Roger will do a piece about it in the Burnley Express.