There is a guy at our dojo who has the same exact problem with a co-worker. There have been lots of suggestions about how to deal with it but none of them have been very good. Given that this boy is your step son your options are pretty limited. You could simply tell him to cut it out (teenagers always do what you tell them
) and leave it at that. A better alternative (if its feasible) is to get him into your class. He might be 'attacking' you because he is curious about aikido or as a way to connect with you (attacks as a form of connection... where have I heard that before?). At the very least it would give him an outlet for his energy.
PS If it comes down to it, don't use yonkyo - it leaves a mark!