Having murdered his uncle and his brother, you have to take this guy seriously.
Firing a missile over Japan IS a serious escalation of the situation. Supposing that missile was to fool the USA and Japan – and south Korea to put in place missile interceptors.
Next, he could fire a missile with a nuclear or chemical warhead : the interceptor would hit the missile, trigger it’s W.M.D. , and then N.Korea could blame the USA for the WMD outcome.