Maybe it's occupational? Like where he lives or works has toxins or allergens that he's specifically allergic to, the sickness leaves him so close to the edge making it hard to breathe, fucking with his insides, so much pressure that he's a millimeter from flying off the handle at any moment?
Usual culprit is mold but it could be heavy metals or sewer gas or something from work.
My wife and I lived in a mold house and it was literally just fits of depression and then fits of rage. We physically fought on a couple of occasions which is very out of character for both us. Since moving the depression went way down and the rage completely disappeared.
I mean it's most likely mental illness or a personality disorder, but it he lives in an old house not well maintained house with a leaky roof or rotting siding or something might be worth considering
EDIT: I'll also add that my wife is schizophrenic and living in mold definitely turned the dial on her symptoms up to 11. Shit had us going crazy thinking a ghost was haunting us. If you had never met us before or seen us after the mold house you would 100% think we were severely mentally and psychotic