Mental illness?

Hello,

My little brother has been acting very strangely for a few months now. He complains of extreme nervousness; he's constantly nauseous, his hands sweat profusely, and he breathes very deeply and heavily during these phases. It feels to me as if his body is giving him a warning sign about something… Work seems to be bothering him a lot, too, as he always comes home completely exhausted and would rather go to sleep right away. In his private life outside of work, too, he behaves like this on the weekends; he doesn't feel like doing anything, and he's nervous and nauseous when we're out and about, or trying to get somewhere by train or bus.

Seems like a psychological problem to me because he doesn't tell us anything and keeps everything to himself without talking about it.