I'm surprised to hear the vet wants him to throw it up.
Any signs of him having chewed it and then eaten it??? It's a pretty long, large item to go down whole and it could get lodged in his throat coming up! Vomiting usually starts not long after being given the hydrogen peroxide. I wouldn't leave him alone until you are sure he's done vomiting. From my experience, with items such as that, the vets wait a day or so to see if it's pooped out while watching for any signs of discomfot. Poor pup! Hopefully the vet will have further advice. Prayers for Boxer boy...Please keep us updated!