You could probably put a Nerite in with him, as they don't leave bits sticking out to be nibbled or bitten off, as long as you don't mind the infertile eggs they tend to deposit around the tank.
Personally, I think the trade off's worthwhile, but some others don't.
The big thing is that you then have a very attractive algae-eater which the betta can't get at to harm, although if they ever do fall on their backs, they can't turn over by themselves and will have to be picked up and placed right side up, so that's something rather important to watch out for.
It would be horrible for a snail to be living in a place where bits could be bitten off any time...