**Disclaimer b/c I just realized this is a 12 week old PUPPY** I would call the vet and see what you should do. I have limited experience with puppies and zero experience with puppies swallowing large bones.
This WAS my original post, assuming it was a large adult dog:
If he swallowed it whole, I would worry less about splintering and more about blockage. If he actually chewed it, I would worry about a cooked bone splintering. Of course you should NEVER EVER give a cooked bone to a dog...EVER.
Keep a eye on him and take him to the vet ASAP if he shows any signs of discomfort (pacing, whining, unproductive efforts at poopig or vomiting, abdomonal tenderness). I'd keep a close eye on his poop for the next couple days for any 'evidence.' More than likely, it will pass. One of our greys swallowed a whole raw chicken leg once

and it passed fine. Of course, he no longer gets chicken legs
