Gambling.
If you are lucky then you will grow you bitcoins in leaps and bounds but always remember that gambling is a game of chance, so odds of losing is high than winning so you might end up losing your coins because of gambling and I don't think there is a gambling site that is designed to make you always win, is there?
I think maybe he is an owner of the dice site or maybe he always lucky and win on gambling, because as far as i know that there is only 50:50, you will win or lose your money and i never saw people who have potential income from gambling except owner