Is my kitten underweight?

We just adopted a new kitten last Thursday (five days ago) and the breeder told us he was 12 weeks old. But I noticed on his immunization card that his birth date is written as 5/27/2011 (17 weeks ago). Then today the vet said (by looking at his teeth) that he's 16 weeks old.

So, ok, the breeder lied or misspoke or something. But, I'm just realizing that he weighed in at 3 lbs--isn't that a little underweight for 16-17 weeks? The vet didn't seem concerned, but now I'm wondering if he isn't eating enough.

He has free range of the hard food and fresh water 24/7, and I'm honestly not sure how much of that he's getting. Morning and night I give him some soft food, but he barely eats any of it, and only if I leave it sitting out for several hours. He'll come by and pick at it once or twice before I give up and throw it out. I'm lucky if he eats half a (kitten sized) can in a day.

I'd love any advice you might have for me. Thanks so much for listening to the worries of a new parent!

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!