
In a way it's 'lucky' these are missing from a back leg. We had one, a Basset, who was born with a similar abnormality but on a front leg. After much discussion (and heartbreak has to be said) with our vet, we all felt it would be kinder to pts - the reason being Bassets carry so much weight on their fronts and he'd definitely have been badly affected had we let him grow up. This was a 'Sectioned litter, and we all noticed it pretty much immediately so let him go there and then. I'd not have felt happy to sell him, and with our small show/breeding pack, and him being a male too, keeping him as a pet wasn't really doable, especially if he was likely to be badly handicapped.
How strange that breeder hadn't noticed the problem with your puppy. I'd suggest you do see what your own vet says but being a light(er) dog, and with a rear foot, I'd suggest he'll be fine going forward.