Does he have a medical reason not to be neutered?
My first reaction would be no....he might be laidback with no female nearby, but once she comes within sniffing distance it's likely he'll become a horror! :shock:
I know one bun that isn't neutered due to health reasons, and he's bonded with a spayed girlie bun. He has the odd hump, but doesn't have any hormonal aggression, or any pent up sexual desire. He had a stroke at some time (his face is lopsided) so maybe that affected his hormones.
You'd have to ask Parsnipbun...it's her Parsley.