My crush is moving!
Imagine if you do ask him out, and he says yes. Will you be able to deal with his moving away? If yes, then go for asking him out. What your friends overheard may not be the total truth, so you have nothing to lose by asking him out. You won’t be forever embarrassed if he rejects you because he’s moving away!
But it might be harder to move on from him if you actually do date and he leaves. While you can get over a crush by just finding someone new to like, getting over a boyfriend is more difficult. And long-distance relationships are possible, but a phone call or Skype chat isn’t the same as seeing someone in person. Plus, you might consider that he might not want to start a relationship before he moves.