To be honest, you really need to get him into a bigger home. He might survive in a half gallon tank, but he won't thrive as you want him to.
Solutions to Ich would be a partial water change, raising the temperature, dropping the water level because of the higher temp to allow the betta to breath easily as the heat may make him a bit lethargic. Once that's done, add in the appropriate medication for treating Ich. However, you aren't going to have an easy time trying to treat half a gallon of water with medication - more than likely you'll harm the fish in doing so with overdosing.
-shrug- Just my input in it, maybe someone else will have a different opinion.
