Bipolar is a psychiatric condition when people suffer from extreme mood swings or something like that.
You only stated that all of his posts were negative, not stating that one was negative:
"Man this mod sucks"
And then the next is:
"NEVERMIND I LOVE THIS MOD IT'S GREAT!"
Then:
"WHY DOESN'T MY MOTHER LOVE ME?!?!"
Or something, but then again, your brother is the doctor