If you keep him you're going to have to grow him some algae which can take awhile. Until then you'll have to supplement his food with algae tabs, blanched spinach, and other veggies like that, then hope that it decides to eat some.
I think you could keep one in your 2.5 gal with the betta, but you're really going to have to stay on top of water changes since they do seem to put out a lot of waste.
One other drawback is that if the snail is a female it will produce eggs, although they will not hatch in a freshwater tank. I got really lucky in that my two seem to both be male.
Could you bring it back to the place you got the plants from?