Danny, your vet might be right if it was an older dog who had been sitting in a hard plastic bed without a mat - but for a 10 week old puppy, it doesn't sound quite right as their skin is a lot more supple and shouldn't stay still long enough to get pressure sores like that.
I would get a 2nd opinion from another vet if I were you - However, I would wait a week or so first, just to see if the hair starts to grow back.
It could be evidence of an injury or burn - but again, this wouldn't have healed properly in his short life. It is strange that they are in the same place on each leg though - a dog nipping hair out wouldn't be that specific surely?
Hmm, I'm flummoxed to be honest... But as long as the dog isn't in any aoin and it isn't spreading, I wouldn't worry about it too much.