Thank you for saving him from a sad situation.
He may be bored with being shut in. I’ve never heard of 10 days of cage-rest for a boy bunny after a neuter (unless there’s been complications, of course). It seems like a long time to me.
Neuter/spaying can increase hunger in animals and make them put on extra weight. It seems a little soon after the procedure though, but it is possible that it is an effect of hormone levels changing.
If he’s not putting on weight and he’s very hungry, you could increase his food. He’s still young and now he’s recovering from surgery, so he may need a bit extra. More greens, a few extra pellets. But he needs his hay, bunnies do. It’s sometimes hard to make them eat hay if you offer yummier food. You could offer some hay in a separate container if he doesn’t want to eat it out of the litter box. If this extra hunger goes away or not, time will have to tell.
As for bunnies making you feel bad for not giving them more food, you have to use your best judgement. My bunnies f ex often “tells” me I give them way too little pellets and unhealthy treats. In that respect, a bunny can’t always be trusted =)
It seemes he’s doing great though, and so do you.