I would go with a snail. Otos prefer to be in groups of at least 3. That being said, a group of three in a 2 gallon would, in my opinion, be way too much. I would just stick with the snail. However, I have heard of bettas sometimes going after the snails so be aware of that. But for the most part he should be fine with one or two. Posted via Mobile Device
I'd go for a nerite snail, those guys are algae eating machines. But you will also have to supplement them with fresh veggies such as zucchini or algae tabs. I got suction cup clips and use those to hold slices of zucchini in the tank. Great little snails in my opinion. Nerites also won't lay eggs and reproduce in your tank because they need brackish conditions to have eggs hatch.
You will want to up the water changes to make up for the increase in waste production by the snail. You could also get an algae sponge and scrub it off as it grows if you don't want to worry about overloading the bioload in your tank.