hmm... i have the same problem with one of my boys. i gave him an african dwarf frog, and he seems to enjoy watching him, sometimes he follows the frog around the tank. if you've got a 2 gal or more tank, you could try getting an ADF, or a ghost shrimp, or if it's 5 gal or upwards, a couple of cories or platies something. another of mine is in a 5 gal with a few platies, and he keeps himself entertained with them just fine.
if your tank is too small to put other animals in it, give him a "cave" to guard, or something like that. one of mine's got a little terracotta flower pot he likes to hide in sometimes.
i've got my tanks next to each other, and every day i take out the peice of newspaper between them for a while and let them flare at each other. you could try a mirror as well, but for some reason mine werent too keen on the mirror -- they always ran away from it -- but they do enjoy seeing the other males.
or, try rearranging their decorations. it'll make em feel like they've got a whole new tank to explore.
i also seem to recall someone saying that they cut the bottom off of a camera's film tube and let it float in the tank -- their betta liked to swim through the tunnel.
if i think of anything else i'll let you know.