At least, his mind isn't. Considering his half-Yithian powers, it's feasible that he might have just taken control of another body in secret. After all, he still needs to do something about the Flying Polyps. Even if he was truly suicidal, it's doubtful that Jack's Yithian father would have gone through the trouble of telling Jack about his true heritage and the threat to come only for his son to commit suicide. There must have been some sort of safeguard planted in Jack's mind tailored to this situation.