Definitly possible. I litter trained an 8 year old bun and her 5 year old daugther last year. For the majority of their lives they lived in a tiny wire cage, and have never been litter trained before.
It took a good three weeks or so of daily accident clean up, but they got it. I took all soft things out of the cage, but dirty in the litter boxes and cleaned the cage out with vinegar about three times a day. They would pee outside of the litter box, and I would wipe it up with paper towell, put that in the litter box then mist the area down with vinegar water (50/50 mix) and wipe that up.
It was frustrating at first, but once they got it they never made a mistake again. They are my best litter box users now.
It will take patience, and the bunny should be neutered for maximum effectiveness. And no your guy is not too old to neuter if he hasn't been done already.
--Dawn