If he has a psychiatrist, you should report this right away. If he is on meds, this should be reported to whomever is the prescribing doctor. While it might be "just" behavioral, it might not be. What does your son say, afterwards? Is he aware of how long he was "frozen?" Is he aware of what was going on around him during the time he was "frozen?" Don't assume he's being stubborn. He might not have much control - you should try to find out from him what he's experiencing, and get professional guidance. There are some people who suffer from catatonic symptoms during adolescence, and you should not ignore warning signs, if they are presenting themselves.