FYI, becareful with those plastic plants, it's pretty common that they will rip your betta fish's fins since their fins are pretty fragile. I'd recommend getting silk (part material) plants ^^ they're less of a hazard, but still make sure there's no pointy pieces of plastic sticking out anywhere! Dx;
^^ If you're worried worried about temperature, a thermometer is a worthy investment! :) If your tank came with a light, the light will often heat up the tank on it's own, so I don't need a heater for my tanks! (I actually have this problem where the light alone overheats some of my tanks!) And if it's an open top, take a piece of plastic wrap and put it over the top and poke a few holes in it to let air through. This will retain the heat and prevent the water from evaporating too fast.
^o^ But he should be okay! Bettas can survive at room temperature.