Could also be the Kraken. If your guy is traveling at a phenomenal speed looking like his going out of the solar system, then you just had your first encounter with
The Kraken got him and there's no way to reclaim him.
If he is just leaving Kerbin's orbit, then he will enter an orbit around the sun at a slightly diffrent orbit than Kerbin itself, and since they don't die, you have all the time in the world to develop the appropriate technology and skills.