Betta's are solitary fish. VERY solitary. He would be much happier without tank mates. The only thing in my betta's tank is a nerite to help with algae and an MTS snail and pink Ram to eat leftovers.
Bettas aren't solitary fish. In fact bettas love being around other fish. I took my male betta out of the community tank and he got all depressed and stopped eating. Bettas are very social fish and do like to interact with other fish. Give your betta a snail to hang with. They seem to like snails. Also if you do upgrade to a 10 gallon some good tankmates could be very peaceful fish like platy's. I keep mine with platy's and he seems to like them. He loves the pleco. He never attacks him but he's always following him around and eyeing him.