You'd do well to measure him, its always helpful to have growth documentation. As far as weigh goes it sounds totally off, when mine was 36 months old he was 9.6'' & 7.2 LBS & @ 48 months he was 11.3'' & 14.2 LBS.

Your correct about goldfish feeders from pet shops they are a bad staple for any species. I'd not be too concerned with having fish in his diet I've found that catfish nuggets work great (chopped fillet).