Should I not feed my cat while treating for stuck hairball?

If he is hungry, feed him. keep an eye on him but don't deny him food. He knows when he is or isn't hungry and if food is making him sick or he is full, he won't eat. Your hairball treatment doesn't require fasting. If it did it would be printed on the container or instruction sheet. You could try growing a container of grass for him. Let him have a little chew on the grass once in a while. It will induce vomiting up hairballs and they eat grass naturally anyway. for his safety and long life never let him out, make him a stricyly indoor cat.