Sorry to hear that Toby needs surgery, I wish him a quick recovery
Maybe you could get the pup, and if Toby seems bothered or upset by the new addition, you could keep the puppy in another room until he's feeling better? He'll have to be confined for his healing anyway, right? Get a couple of ex-pens to keep both the kiddos in so that they can see each other, but won't be able to play (until Toby is healed up)
Good luck!