You say he had some extra loving after his op - did this include being allowed up on the sofa whenever he wanted?
If so, you can't expect him to understand why the rules have suddenly changed.....
He's not "challenging your authority" he's discovered that the sofa is very comfortable, thank you very much, and doesn't see why he should move - there's nothing in it for him to move off the sofa!!
Rather than getting cross and confrontational with him (which runs the risk that he will start protecting his comfy spot!), train a reliable "off" that way you can ask him to get off the sofa whenever you need him to be off - however if you don't want him to get up there when you are not there the sofa has to be 100% out of bounds... dogs can't understand complex rules.
He is probably hitting adolecense, so will be pushing the boundaries you set, but it is very important that those boundaries are clear and consistent!
We knew we would never succeed at being consistent enough to keep Honey off the sofa, so she can get up as she likes. Though to come up near us, she waits for an invite, and she will get off if we ask her to - empty sofas are fair game, though, and any recently emptied spot is the best place to be (we do a countdown to see how quickly the "seat thief" strikes!!!).