Take him to a vet. His first vacc would have been (in normal cases) at 6 weeks old. If he's not eating right he may not be feeling good. Perhaps try switching his food, slowly, to a puppy food. Otherwise try soaking the food for a bit before giving it to him, to soften it up... If you do it with warm water it might make it smell a bit more so he's more interested. But get him to a vet, he needs a check up for weight, worms, vaccinations and general health too.
good luck, he's very cute!
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