While flying, my son complains of ear pain (and the popping isn't too pleasant either). Is there any way to reduce that pain/avoid it?

I have been facing this for ages now. This happens due fast increase/decrease in altitudes which results in sudden change of pressure. I personally feel the pain even in elevators of high rise buildings. haven't found a full proof solution for this but in flights i try gulping water while facing upwards. Its more like you are swallowing water. Many recommend chewing hard sugar candies but it might be dangerous for kids as they might swallow it resulting in choking. Water does help me in reducing the pain. I also puff some cotton balls in my ears.