As long as your son is learning kickboxing in a legitimate club with proper instruction and regulations, it would be no more dangerous than any other competitive sport such as basketball, soccer, or football.
Kickboxing is an aggressive sport that will involve hitting and kicking and getting hit and kicked. It will improve his strength and flexibility and endurance. He should be using protective gear to prevent serious injury but there will be bumps and bruises anyway. Talk to the instructor about how to avoid injury and make sure you are comfortable with everything.
I took a kickboxing class and we never hit/kicked people--just the air. It was fun and empowering! Your instructor should be able to teach you the proper way to punch/kick so your don't do unnecessary harm your knees and whatnot.
How can it be good for you? Obviously it is a great cardio exercise, but without proper head gear you are setting yourself up for Parkinson's disease in 20 years.
