Since you got the kitten from the shelter, it could be that they were estimating his age, rather than knowing exactly when he was born. Like all living things there is some variation from individual to individual. If the cat was closer to 7 months, then I might be worries, but two weeks late coming out, I wouldn't worry that much about it, especially if the Vet wasn't. If he doesn't lose them in the next or two month, ask the vet if you should get x-rays to see if his permanent teeth are coming in at all.
If he's already 8 lbs. he's going to be huge when he is full grown. The 20 year old Spike is only about 7.5 lbs. and Hobbes is about 12 lbs. Hobbes is very tall and long, I guess kitten chow was good for her.