I noticed today that when he walks (he is 12 weeks old) his back legs don't follow in line with his front legs (for example, if his front paws make marks on the floor, his back paws would be to the side of them, not level.) Is this normal? It's on and off the lead.
Genetics and body shape/leg length play a large part in gait, as the structure of the dog - whether it's tall, short, wide, slim t etc affects foot placement. eg
"Wide-set dogs with low centers of gravity, such as the bassett hound, do not single track. These dogs normally move with a pronounced lateral roll of the body that is considered abnormal in dogs with longer limbs."
So dogs like Bulldogs, Corgis etc don't single-track, simply because their anatomy prevents it.
Yep, if this is the puppy who had a strange-walking sibling, get some veterinary attention. It's much better to get this now, while he's young (although obviously it would be better not to need it at all).
Mention to the vets the hipscores of the parents, if you know them, and that the sibling has walking difficulties, so that you get the best course of action.
Any treatment will have a better prognosis now, rather than as he gets older and things get more worn.