Unfortunately this is an operation that should preferably be done when they are younger, so I think you need to ask yourself what the benefits are for him. If he can have a friend once he is neutered, then yes it will be worth it, as rabbits need company. If he is frustrated and constantly humping things, then yes it will be worth it as he will be less sexually frustrated. But if these things dont apply, then personally I would not get him done - because he is getting old and there is little benefit to him. After all, this has to be for the rabbit, not the owner.
I doubt you'll see much change in behaviour I'm afraid, as he has been left for so long most of the behaviours will have become ingrained and no longer purely hormonal.
Yes it will hurt him, with pain relief he should be back to himself within a day or two, but he'll take around two weeks to heal. The success rate I'm afraid would be very hard to calculate not knowing what his gut, lungs and heart are like. Normally it is extremely low risk, but obviously the risk of anesthetic will increase with age.
edit: by the way, thought I'd mention that there is no link between being a 'snuggle bunny' and being neutered