Anyone else’s kid a lot smaller than kids their age

So my kid is 7 he is 39 pounds. He is 46 in in height. I took him to the new doctor and she looked at me with concern. I said I know he is underweight and I’ve had his bones checked and his blood tested. He has been the same size since like 4 years old with the exception of gaining a pound or two since then he was 36 pounds…I give him whatever his little heart desires and I have a 5 year old they are the exact same size minus two inches. When I took him to have his bones checked he was 2 years behind bone wise. But they said he would grow into it. And he hasn’t. Anyone else?