First hope you and your son are not seriously injured.
What you described is an all too common occurance here where so many people ride ATVs. Many Doctors here hate ATVs, due to the injuries they see. In the villages that is the only transportation for many families. You will see six and seven year olds driving them.
One of the first experiences I had. I had bought a Suzuki two wheel drive. My wife was on the back and like you we were climbing a small hill. Suddenly I realized the machine was rearing up and going over backwards. We bailed off, but I pushed her to the side, out of the way. In doing so I got caught underneath the machine. Nothing broke, but I was sore for many days. Every month or so we hear about someone getting killed while riding ATVs.