Can Chin Implants Be Knocked out of Alignment?

Yes, chin implants can shift if trauma occurs early in the recovery process.  As each week passes, it becomes much more difficult for this to occur.  Ultimately, the chin implant should become integrated with the body so that shifting can become more difficult.  

Eric,
In general, chin implants are very stable. However, early after surgery, before the body has fully incorporated the implant, it is possible for it to shift. Even over time, if there is significant force, it would be possible for it to shift.
My advice would be to hold off on the surgery until your son is not participating is such activities.