Actually, my son with the metabolic issues dropped off the scale at 6 months and his twin (who had been over 2 pounds smaller) passed him up. He started off at 6 lb. 9 oz. compared to his twin who was 5 lb. 4 oz. Meaning for my size and their twin gestation he was a good size baby.
I will also say that I greatly increased Andrew's fat. He was eating way more calories than he needed and still not growing well. (He did grow some).
Turns out he can't metabolize fat. He also lost skills over time.
My point is that with unknown issues that could be metabolic in nature I think you want to keep the diet not heavy in any thing--fat, protein, carbs. You want balance.
Yes, I think there is an underlying issue or it would make a difference.
I wish I knew what it is.
One thing: is there a family history of short/tiny people? Because kids can go to their genetic predisposition around 6 months too.