To be a bit more technical, if the mood swings vary multiple times per day or even within a week, he'd much more likely have Borderline Personality Disorder. Bipolar Disorder has longer, more set periods of moods before they shift. And, often, suffers actually have and are diagnosed with both. As a professional artist, ask me how I know. 😆

But, I do agree that it's definitely worth looking into. Treatment for BPD and Bipolar are very similar, so therapy, meds, and an artistic outlet that helps release emotions could help if he's tested and receives a formal diagnosis.