Go to another vet (one the specialized in small breeds) and let them know of your concern, even get x-rays if needed...it's good to get this "investigated" now...also if you have any friends that are lawyers or are familiar with how that kind of thing works, explain your problem, the petshop might be liable for more than you know...
I hope you can keep the little guy...it is sad to even think that there is people that just don;t care
P.S. Mojo was a very very calm dog, his first bark was when he was almost 6 months of age. He didn;t play much...just cuddled and gave kisses...this only changed when we got our chihuahua, them he had a playmate now he will play with anything that moves (but he is not hyper at all) so this also has to do with his own personality..