Was the bipolar issue really nailed down as an accurate diagnoses? Does he really have a problem with seratonin uptake? If that's the case, it may help him to take a supplement of essential amino acids so his body always has enough pre-cursors. If it's not the case, then it could well be a combo of candida, not enough friendly microbes, etc.
Unless it is definitively a seratonin=uptake problem, it's a systemic issue. Either way what you're having him do will help. :)