Did the Vet say he was healthy overall? Kitten's weight is usually about a pound a month, so at 4 months he should weigh about 4 pounds. But that's just a rough way to tell, and each kitten is different and three pounds at 16 weeks isn't bad if he seems healthy otherwise. I just adopted a kitten last week that weighed in at three pounds, and he was just 16 weeks old. His birthday is June 15th, so he won't actually be 4 months old until October 15th, so he still has some time to gain some weight before the 4-month mark..

Does he have worms or intestinal parasites? They can affect a kitten's digestive system, rate of weight gain, etc. The Vet should have checked a stool sample.

You say you got him from a breeder? Is it a REPUTABLE breeder? There are a LOT of "breeders" out there who don't have the animals' best interest at heart, and don't take proper care of their animals, etc. If there is some discrepancy with what you were told the birthdate is and what is written on the immunization record, it could very well be that this is one of those situations where the "breeder" is not reliable, reputable, honest, or qualified to be a breeder! Often times those animals have underlying health problems. I hope that's not the case with your precious little kitten. Please keep us posted as to his progress!