He looks good, what is he flaring at? If he is really aggressive towards anything that moves, you might not be able to put anything with him (maybe a snail since it's a good size tank). Otherwise, if he tolerates them, he could be with any of the following (not all in the same tank, it would be too crowded):
Usually safe:
White cloud mountain minnows
Snails/shrimp
African dwarf frogs
Cories
Kuhli loaches
Danios in a large enough group (so they pick on each other, not the betta)
Female guppies, or non-fancy males (a few bettas will tolerate fancy male guppies, but most think their tails look too much like another male betta)
More dependent on temperment of betta:
Small, friendly tetras or rasboras (rummynose, black neons)
Platies
Cherry barbs
Things to avoid with a betta:
Any other betta
Gouramis/paradise fish
Nippy fish like serpae tetras or tiger barbs