I would check the local laws in your area but what i would do is.
Make sure he is old enough to wear a properly fitted (approved) helmet.
Make sure he is old enough to understand How to ride as a passenger.
Make sure he is old enough to know how to not only hang on ,and how, but also that he understands he needs to hang on all the time.
I know that some kids, Most kids, lose their concentration quickly. so age intelligence could vary.
Remember, YOU are the parent and our kids will do what we tell them (until they are teenagers at least) so remember make sure YOU use good judgement.
Just my ideas