Where did you buy your pup? Why do you think he's not Wild Boar?

Not having seen your pup but I would tend to agree with your vet about filling out in the chest area. As you stated he was underfed and very thin when you got him. After several months of proper feeding, exercise and TLC I'm sure he will round out like every other Doxie. He just needs time. Good luck!

JMHO