The best way to do this is to simply add them as a friend. If they're online, they'll get the request. This will make it loads easier. However, if you don't have their in-game name (call sign), then it will be considerably more difficult to do. If that is the case, then just play around until you find him again. In this case, though, you could give some information to others (their behavior, your call sign, stuff like that) so they can find him and give him the message you want him to have.
I hope this helps.