Hi and welcome to YT
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Honestly, at 10 weeks old and at this dangerous size - he should really still be with his Momma. Ethical breeders will not even allow their dogs to be released until they are at least 12 weeks and oftentimes at least 2 pounds.
If he is truly lethargic, then he really does need to see the vet again as lethargy is not normal in puppies.
You and this tiny pup are in a *very* vulnerable situation here, and you are going to have to be extremely diligent in staying on top of keeping him alive right now.