My puppy is shedding, what could it be? please help!  

Could be a multitude of things. Have you recently changed his food? Has he been around any other dogs? Have you checked him for fleas? Are the spots where he is losing the hair red and warm?

He may be having an alergic reaction that is causing him to lose his hair. This would be my first guess. Try putting him on a bland diet (usually lamb and rice is best) puppy food of course. If he is chewing or scratching real bad and you see any evidence of fleas, give him a bath to treat them. He may be alergic to the flea dander. And get him to the vet as soon as possible. Your vet should have someone that fills in for them when they are away or there should be some kind of emergency vet you can take him to.

Also if you had another dog die from similar causes please make sure that the vet knows this. Your puppy may have a sarasoptic mange mite that is attacking it. The mites can live in the carpet or on old bedding from your previous dog. There is medication that can be given to treat this but should be handled immediately so that the puppy does not get any worse.