As Romeosangiovese said at 12 weeks they should have completed their most important time of socialization. The puppies should be excited to see you and want to interact with you. They should not be afraid of children, other animals and some strange noises. These are important things to look for, because if the breeder hasn't socialized them well, it will be very difficult for you to socialize the puppy now. Of course, even if the breeder has done their job well, you will need to continue some socialization on regular basis. If you have fallen for this sheltie pup and him for you, go for it!!