I was pretty much like that when I was 20. Do you notice your mood change on it's own or do things that happen in your life trigger mood changes? I have low-grade depression most of the year compounded with chronic anxiety. Let him work on whAt he sees then work towards a dx of whatever.

You can get racing thoughts and looped obsessive thinking with anxiety which can switch moods rapidly. For instance I watched blackfish (this documentary was pretty triggering in hindsight) and became obsessed with sea world controversy. I couldn't even sleep well because I couldn't stop thinking about it and I kept rereading the same thing over and over in a kind of compulsion. Does this sound similar to anything you're experiencing?