everything bluish said, and do regular water changes.
depends on what size of your tank is. if it's in a 2 gal tank, i'd say a weekly 50% water change. if anything less than that, better off do water change more frequently (like 2 / week). use aged water for water changes. sudden change in temp will likely to make them sick.
other than that, i think they LOVE live plants. it'd be great if you can give him a live plant. watch out though... rotted live plant can cause water pollution which isn't good. i use java ferns in my tank, and that seem quite good.
if he's alone (with no betta neighbour), you may want to show him a mirror to let him flare at it once in a while. i just thought that they won't get too bored that way