My son is 4 and he is still into some 2-3 t’s. He was 3 he was still wearing 24-18 months clothes. I know that I am not that tall and neither is his real dad. But, I do have a brother who is 7 foot. My son has always been little though. When he was a new born he had to wear preemy outfits. So he has been small since then. I have been giving him some vitamins  and I think that he lives off of choc. Milk. so we have been buying him the nestle quick powder mix and he loves that and its good for their bones as well. I wish your grandson well and hopefully he will grow up to be a tall child and be a basketball star!! Good luck!!