Definitely. A five gallon is only really suitable for one fish, unless it's the micros I mentioned. Bettas are solitary animals anyway and don't typically do well with tankmates. Male bettas can be extremely aggressive, in particular, with anything they view as competition. You could also have a few large shrimp in the tank with him, as they don't add almost anything to the bioload. Not saying you wouldn't see aggression, but shrimp and snails are the only real success stories I've heard of tankmates for bettas in 5 gallon tanks. In a 10 gallon, there are a few other possibilities.