As far as I know, he does not manage money directly. He is based in London and advises Boston based GMO on their (global) asset allocation. GMO manages a lot of money (like 100 billion or so).
He writes insightful letters (also a few good books), always with a bearish touch.
GMO has been right at the major turning points, oftentimes early. Their 7-10 year forecasted stock returns are probably as accurate as anyone's.
Just my 2 cents.