You can keep him temporarily in any clean container, just change half the water every day or so while he recovers. I bred bettas for a while and they did fine in smaller containers although I generally preferred to give them tanks with plants. Bare bottom tanks are easier to siphon the poo out of too, especially for temp quarters while your tank cycles.

I kept single fish in beanie baby containers(about a quart I think?) for quarantine with a few sprigs of plant. They stayed quite healthy.