Increase gear ration on child's bike?

My 6 years old son has a disease that causes him to have very low muscle m. He wants to ride a bike, but his bike is too hard for him to pedal. It moves freely, but with his weight, it is hard for him to get enough power to move. I'm wondering if increasing the gear ratio would help him, but I don't know if this is possible for a child's bike with a fixed gear. I tried to find a bike with different gears, but it doesn't seem to exist. I don't know what I'm really asking, I guess, just wondering if anyone has any suggestions to help me get my son on a bike or increase the gear ratio. Thanks