I have no idea. It could be a chemical imbalance (which tend to be genetic like schizophrenia or bipolar) or it could be a personality disorder. Like borderline or narcissist. Or a combo of a PD and a mood disorder.

He'd need to be seen by a therapist or psych doc for a long term period (probably 3-6 months) to get an official diagnosis. Most people are diagnosed with bipolar in their 20s or 30s, but there are exceptions of earlier and later. Whoever he sees will first be working on his behaviors (or acting out) after they build a trust and from talking about those periods of time when he feels on top of the world vs. being more meek and kind, the therapist or doc will have something to report to your parents about how to best treat the differing personalities.

I had different personae in each class in high school and on different days depending on my mood, but I don't think I ever treated people like **** all day long. I was just quiet and withdrawn in one class, hostile and irritable in another, class clown in yet another, teacher's pet in a fourth, etc. Perhaps I just react differently to varying environments, but then, I would have both depressive and manic days with the exact same people in similar situations.

Hope this helps some. But your parents probably need to get him an eval because the sooner a personality or mood disorder are detected and diagnosed, the better his chances are for getting the proper treatment, reversing behaviors and ways of thinking, and in the end, survival and leading a happy, productive life.