Bettas do pretty well alone in a 1 gallon, like you have. They will take oxygen from the surface, as you might see him gulp air from the surface of the water. He might also build some bubble nests. If you decide to get him tank mates, I'd suggest a 10 gallon and consider 1 gallon per inch of adult fish. Don't forget to cycle it first. Some good tank mates are non-aggressive communal bland colored fish, as colorful fish might seem like there is another betta, and they will fight or attack anything that looks even remotely like a betta. I have a female betta with 2 African dwarf frogs, and they do well together.