Definately need a bigger bowl and a proper heater. Using a light for heat is not a good idea. Firstly, it's very inconsistent in heating. Secondly, fish need dark time just like we do, and as soon as you turn the light off for them to rest, the temperature will drop off, leading to even more inconsistent water temps. A half gallon bowl is not a sufficient home for your betta if you cannot maintain a healthy temperature for the fish. Do him a favor and upgrade and heat, it's worth the extra room on your desk.