Son claims to be full all the time and doesn't eat much...

[LondonChillaxer]

We are having a real dilemma with our 6 year old son now. He has always been a fussy eater, being very selective about what he eats and when he does usually not finishing his portions. When we persist he retorts that he is full. We had hoped that his appetite would grow with him and were relaxed about it. However, this issue has continued to the extent that he would now have two spoons of cereals in the morning and after a few bites at lunch time insist that he can't eat any more.
He is in good health and very sporty which makes us wonder where he gets the energy from! He is on the shorter side amongst his peers and we are trying to use the argument of 'you need to eat to get bigger in size' but it doesn't really convince him.
We are thinking of conferring with our GP about this.
But would be grateful for any suggestions/ personal experiences.
Many thanks

Does he ever complain of tummy aches? Is he burpy? Just wondered whether lactose intolerance causing gassiness/feeling of fullness might possibly be an issue??

This was something ds1 did.

He had an undiagnosed posterior tongue tie which I discovered when he was 6 yo. This meant his swallow pattern was wrong and he was swallowing down a lot of air.

He was also extremely fussy because there were many foods that he had trouble chewing and swallowing.

Thank you CrushedVelvet, mawbroon - I had not thought of these.
He does complain at times of tummy aches (though only occasionally). Haven't noticed him being particularly burpy.
I will certainly have him checked now.
Thanks again.