I agree with everyone. If it were me, I wouldn't send him back for fear of what this person who shipped you a sick, too tiny dog in the first place whould do with him. This "breeder" is obviously not credible, so who knows what might happen to him, if he even survived a plane ride. I very highly doubt this puppy was healthy when they sent it (some of these people have shady vets "on their side" to sign health certificates, sad but true)and even if he was, he was WAY too small and young for a plane ride.
That being said, I would 110% pursue legal action. You said you paid alot for him, I have no doubt you did and I can guess at the price. As was said, there are "puppy lemon laws" that vary by state. After what you paid for him in addition to the large vet bills you no doubt have and will continue to have at least in the short term, it would be absolutely worth it to persue getting every single penny back and maybe even then some for the vet bills. I've never persued something like that, so I don't have specific advise other than to start looking into it. Also, hopefully you persuing a refund and legal action against this person will possibly help someone else from being ripped off and going through what you are.
It's so very sad and unfortunate that people like that are able to make money and cause such trama to these poor dogs and their unsuspecting new owners. I stick to the basic rule that I would never ever purchase a puppy sight unseen, especially if it were going to be shipped to me, they're living animals, not merchandise and frankly, I don't even understand why it's legal.
I feel bad for you and your poor little puppy
, hopefully he'll come around and be ok. Sounds like you are able to handle the expense of his vet care, so keep that up, follow any advise from your vet, and absolutely fight to get your money back. If it were me also, I'd make darn sure to spread the word of how awefull this "breeder" is, someone's reputation would be ruined at my hands to prevent it from happening again. There's a very good chance it was a puppy mill and should be shut down anyway.
Good luck sweety, hope everything works out for you.