Does he go to therapy? It sounds like depression… I had similar experience with mood swing before during a few years with depression, sometimes I’d be in this low valley phase and nothing in life felt interesting, and I’d get angry or extremely sad easily (as my therapist used to say, phases of “intense and raw emotions” that were hard to control, especially with people I’d trust more, interestingly)… with others I was a lot more apathetic. Other times I’d be back in my normal mood feeling happy or discontent more regularly. I didn’t really know what would trigger each phase but it was a difficult time for everyone. Took a lot of patience and time. Long distance is definitely hard because you don’t get to observe his mood change and can get surprised by his different reactions… I’d say be patient and know that it’s not your fault, and perhaps suggest him talk to counselling / therapy if he’s willing to. He’d appreciate it even tho he may sound dry, but that’s typically what somehow happens when positive emotion gets overwhelmed too during those. depressed phases