Usually bettas prefer to live alone. You *may* get away with adding an oto or 2, or a couple of corys, but have a back up plan in case he attacks them and it goes wrong weeks down the line. It frequently does, honestly, but the do love your company and your company alone.
I keep all 5 of mine alone, but I could not trust them with anything else, they're very fiesty haha!
EDIT: Or divide the tank and have 2 bettas with 5 gal each lol :;a2
There are a bunch of threads on Betta companions in the Betta section of the board. I have my 3 all with small schools of cories and they do fabulously. Others swear by snails. Alone is good too as they are such cool little fish.