Tell him to stop being bipolar and start being bi-winning.
(sorry couldn't help myself.
)
Seriously though, I think I had a friend like that. I can't say his moods changed constantly but he had some real issues. Had a very narrow set of fields he was absolutely OBSESSED with (to be exact three subjects: Music, Sports, Video games, period.)
He either hated things, loved them or disregarded them completely. We were really good friends for many years but as I grew up I grew sick of him and we both parted ways.