Your puppy is too young to be away from his mother and littermates. Physically he will be able to survive, but mentally he should be with them for another three weeks or so. Depression is normal if you take the puppy away at that age and do not have another dog for him to spend time with.
You should first take your puppy to a vet and have a thorough physical. If he has not been dewormed, he will have a heavy roundworm burden and will need to be treated for that. He may already be anemic.
He may have already developed parvo or some other infectious problem. I cannot tell you how healthy he is based on a short description.
If his health is okay, or after you have had him treated, you will need to spend adequate time with him to get him over the depression. This includes being with him at night, not leaving him alone all day, etc.