in regards to "bulking up" in pre-pubescents.. there is plenty of literature out there and basically its not going to happen until you are well into puberty. if a 10-12 year old looks more "bulky" than the next guy its most likely just genetics. i can remember short, stocky wrestler types when i was in 7th grade and all of them are basically the same height they were as freshman in highschool. its the lanky, slow, growing "hard gainers" (which basically just means under eaters in my book) that have the potential for tremendous growth once they are well into puberty. 13 is barely even at the starting blocks as far as development goes. program him correctly now and it will go a long long way into helping him become an accomplished athlete in the future.