Looks good to me, but veganchick is right, keep an eye on the coral. Make sure there are no holes in there he might try to swim through and get stuck.
a 7.5 heater would work. Max would be a 25 watt with a temp setting knob, especially if you're thinking about getting a slightly larger tank.
I would make 1 suggestion. Lower the water level a little. Maybe an inch or two from the top. And does that lid have air holes in it? It looks like a nice lid! But Betta breath air as well, so you need some flow. It's alright if the plants are sticking out the top, or bunched around the surface of the water, it'll give him more areas to hide in. And might give him a good place to make his bubble nest. Mine builds his between the silk plants sticking out the top.
I'm glad he's eating, by the way! Whatever he doesn't eat, you need to take out of the tank. It'll fog up the water really fast!