congrats on rescuing the little one!!
also agree with tonibun. if bun is otherwise in fit and in fighting condition then I would say they're never too old to be neutered. buns have been know to live for 8, 10, 12 years even so he could have a lot of fight left in him, which could be destructive as I've known neutered males to shred anything :lol: along with the litter training being 10x harder, and finding him a friend would be difficult. buns really need to live in pairs and as he could live on for much longer, a friend could/would be nice for him.
if you're in the UK, the RWAF record of all of the vets they recommend:
https://rabbitwelfare.co.uk/rabbit-care-advice/rabbit-friendly-vets/rabbit-friendly-vet-list/
its down to you, and your trusted vet. but I do think he could be done, and I do think it would be beneficial for him and for you, and then he can live out the rest of his life hormone free and possibly with a friend
you might be up against the tide as good eating habits are formed at a young age. however its not impossible. usually I'd always suggest cutting back mostly everything else and just giving them hay, they're not silly and will not starve themselves. I know it sounds "cruel", but eating hay is far more important. also mixing up times you give pellet or veggies would help, as some rabbits try to "hold out" until they know when their next meal is coming.
all the hay here I would also recommend, timothyhay.co.uk, hayandstraw.co.uk, natures own & hay for horses. you can buy it in bulk/bigger bags which always works out better than the piddly little bags in PAH