So he is 10 weeks old, undersized, and has diarrhea, and somehow because of that the advice that he should eat as much as he wants is wrong and he needs to eat less? That really doesn't make much sense imo. If you don't trust other experienced cat owners, vets, magazines, and books, then perhaps what it says on the blue buffalo web site about that food would help:
Feeding Instructions
Up to 20 weeks of age, feed as much as your kitten will eat in 3 or 4 feedings per day...
http://bluebuffalo.com/natural-cat-food ... en-entree/I really think you should look into much more logical causes for the diarrhea, and make sure to get that taken care of without underfeeding him. It may simply be that this is a new food for him? Something different than he was eating at the breeders? It could also simply be from the stress of the big upheaval that he just had in his life, and may take a few days to settle down. Otherwise it certainly could be a parasite (which is very very common), or it could be an allergy to that particular food. In all of those cases though, the answer is certainly not to underfeed him. He really should be eating more, particularly if you are concerned about his size and growth.