Hello & welcome.....Yes, this IS something to worry about....If your dog was an adult, you could wait it out a few days...With a puppy, you CANNOT! Not eating is one thing, but not drinking is another...Is he drinking?
Dehydration can cause a puppy to go downhill quickly.....Vomiting &/or Diarrhea in a puppy needs to be checked out by a Vet ASAP.....
Dehydration in a puppy this young can happen in 24hrs. and can be fatal....I'm not trying to scare you, but to emphasise that this could turn into a medical emergency (If not already).....
Also, ask the Vet about worming, as an overload can cause these symptoms along with no weight gain......
Either way, Take your puppy NOW, to see a Vet......Please, let us know what you find out.....Karla
has he been dewormed ? go and get some chicken and boil it and make some rice and shred the chicken into pieces, that usually helps but if it doesnt you need to take it to be looked at