I agree with Ginger. If your dog only had the surgery this morning, he is still very much a patient at the vet's. Just because he was allowed to come home doesn't mean he isn't still under a vet's care, so the vet needs to be notified ASAP about these new developments. Please don't wait. Even if it turns out to be nothing, it's better to be overly cautious, especially in the first 48 hours following anesthesia, than to put things off and maybe run into complications. Please let us know what you find out.
Ghilly
I would speak to your vet about these symptoms as soon as you can.
The swelling under his "armpit" could be a reaction to the IV line that would probably have been installed for his surgery. No doubt this is an accumulation of fluid, and when the anesthetic wears off properly, and he gets moving around again, it will be re-absorbed. But I would definitely check this out with your vet. And as far as I know, the vet who performed the surgery has a duty of care to do follow-up checks, even after moderately minor surgery.
The "indentation" on his head could be swelling with a "dip" in the middle. Swelling again maybe from fluid accumulation?
(Just to illustrate what I mean...I remember I once fell while climbing on some rocks and hit my shinbone very badly. I kept going but a few hours later was horrified to feel what seemed to me to be almost a "hole" in my leg! I took my trousers off and looked, and what had happened was that there was swelling either side of the injury, and it felt like an indentation, but really wasn't one.)
I do not know for certain, of course. Don't take my word for it! Get him checked out.
You do need to discuss all these things with your vet.