Or even better- start by quietly calling him to the principal's office. Ask him about his clock. Tell him people were thinking he might be wanting to pull some bomb pranks, and that would be a very bad idea. Ask him to tell his teachers the day before the next time he wants to show off his gadgetry. Send him back to class.
Or don't do that, either, because it is not right to assume people are going to pull off bomb hoaxes. (Doubly so when the only real reason for the suspicion is skin color.)
Yeah but you also don't want to be the admin at a school with a bomb where there were "signs a student was creating bombs before he went ape shit". I agree with the color statement, but not addressing the issue at all can be just as bad as an overreaction.