How can you tell if a puppy has been taken away from his mother too soon?

I bought this puppy from a guy who got him and could not keep him due to his land lord. But what makes me worry that he got taken away too early is because he will snuggle up to a big blanket and push his paws on it and get a tip of the blanket and suck on it as if he's nursing from his mom; like cats do as they are purring. Then when he sleeps he does sucking motions as if he's dreaming he's nursing from his mom. He's going on 4 months and I got him at 7 weeks. He's been doing it since i got him. I have other puppies and they never did this so it worries me that he gotten taken away too soon.