Ooh, it's so nice and spacious!
I'd add some more decorations. If you don't want to try live plants, you should go for silk, not plastic, so he doesn't cut his fins. He'd also probably really like some floating plants or decorations, that way he'll have a place to anchor any bubble nests he may decide to build.
Some bettas will not tolerate any other fish, and some get along quite well with a variety of fish, so you can definitely add some but you should be prepared to rehome them if your betta decides he doesn't like them. Some ghost shrimp would be a good test, as they are only about 10 cents. If your betta doesn't try to kill them, you could add some more colorful and expensive cherry shrimp.
Bottom dwellers are also a good bet for betta tankmates. A shoal of 6 dwarf corydoras would make a lovely and interactive addition to the tank. Although before adding any groups of fish, you may want to upgrade your filtration.
The only other thing I would recommend is a top, as bettas have definitely been known to be good jumpers. You can get a nice glass one for around $12 and it has a plastic strip at the back to cut out holes for your filter, heater and any other things you may have running through the tank.
All in all, this is a great tank. You have one incredibly lucky betta and I'm sure he will be very happy, healthy and long lived.