I struggled for a second with figuring out why my character should be alerted to the attack.  Just went with the "Saw it in progress" approach.  For King J, Omega mentioned the distress call was going out to all former and current allies.  Maybe your boy has an old communication device that finally pinged and he goes claws out, ready to take on Skeletor's underlings just as his father did, like a debt to an ancestor to carry on the fight.