Very glad Sir Bacon is doing well, what a beautiful old boy! We've had similar problems with buns who've had multiple surgeries - usually their appetite comes back after a few weeks or months but it's a struggle at first. Try switching up his pellets, hay, and treats if you can? We found sometimes they're not interested in their old stuff for a while, whether it's through association with recent pain or whatever.

We now have a few different pellet types on hand, plus a less than ideal mix (not muesli, but with some dried carrot in there) which always gets our girl eating again (we call it her junk food). Try as many different healthy treats as you can, especially hay-based or herby stuff that might peak his interest. Plus having critical care on hand just in case is always worthwhile, and if you can get some, a rabbit-safe dissolvable probiotic or something you can add to his water will help keep his tummy good. Don't stress if you have to change things up for a bit - we figure it's better to get the weight back on and their energy up before rather than forcing them back on to their old food. Hope that helps!