He’s only been neutered 2 weeks, he’s still hormonal (he could still get a female bunny pregnant) and litter training shouldn’t even really begin to be expected to be effective until his hormones are out of his system (an additional 2 weeks). Rabbits who are hormonal are very hard to train, because they want to mark. Plus, he had those complications with his surgery, so his behavior should be off a bit. I would be a bit off as well if my incision opened up and I had to stay locked in a little carrier to avoid it happening again! Give him some time to calm down.
Buns can be litter trained because we manipulate their instinct to keep their living area relatively clean. You say he holds it while in his cage and the cage isn’t very large. It’s possible he doesn’t want to soil his living area and a larger area would help this.
I would be patient a few weeks, put a couple of litter boxes around the computer room and make notes of where he pees the most. Rabbits kinda choose where the box is, we don’t. Wherever he pees the most, move the boxes to. Once his hormones calm down in a few weeks, be very strict about cleaning up after him, putting all his stray poop into the box and if he pees outside soak it up in a paper towel and put it in the box too. Be sure to clean all accidents up with vinegar/water (50/50) to remove the smell, resolve won’t do it and he’ll still think any area he marks smells like it’s the litter box. Has to be something that will dissolve the pee (and oil from the poop marking), so something acidic. Nature’s miracle works too, as it is specifically designed for amonia based urine removal.
It’s possible you may need to overdo the litter boxes to get him to start using it. For example, when I first started trying to get my two to use the boxes at all as a bonded pair (they totally forgot litter habits when I bonded them), I put down SIX litter boxes in ONE room. They had chosen so many different places to mark, one or two just wasn’t going to cut it. Two entire walls of my room were nothing but litter boxes in every inch. As they started picking favorites, I reduced the number, now we have four and if I go any lower than that, they will just go where the missing boxes were. I also had to block off the dark, covered corners because it was too tempting for them (closets especially). They still aren’t perfect, but they do use the box mostly… and the mess they leave outside is generally directly outside the box, so I put saran wrap down to protect the floor.
Another member once had to line their bunnies ENTIRE area with litter boxes, so the bun could only access litter boxes and not floor.
Moral of these stories: Get creative. They aren’t all easy to train. But, I don’t think any are totally untrainable. I adopted Powder as an adult, he didn’t have a litter box in the shelter, he had a metal grate under his feet and a slide out tray. Stickers was in a cage covered in wood shavings, with no litter box. You just have to get them into the habit of going in the box, they are very smell oriented so it’s key to make the litter boxes smell like their messes (clean litter box is just a bed, put some mess in them yourself) and make sure no other area does.