Is it likely that a swimmer puppy’s legs may never fix?

Isn't it more about the shape of the ribcage?

We’re taking the pup to the vets tonight

Absolutely gutted, vets decided it was in his best interest to put him to sleep as he was in pain and his health was deteriorating. At least he is pain free now:(

So sorry, but this is the down-side of breeding I'm afraid.   I still weep for a little bitch we had who suffered anoxia at birth.  We kept her to 18 weeks but she was clearly in trouble, so we had to let her go.

My sympathies.