He was pretty confident he would lose it all and not have to pay any tax on any winnings this year so he didn't want anything withheld.  I have a client that didn't rack up the winnings that yours does but for years she has brought in her W2Gs and the ins/outs of her slot machine activity at the casino.  I really don't know how someone can possibly sit in a casino that long to dump that much money into one of those contraptions.