There is a "20 Questions" page on that Gambler's Anonymous link.

However I think there is nothing you will be able to do until he recognizes he has a problem. He is going to have to want to get help for himself in order for anything to really help.

Of course you can do your part by not supporting him in gambling... i.e. never loan him money to cover gambling debts, not making it easy to shurck other responsibilities to go gambling, etc. If he becomes angry over this, point out that his getting angry about it is a good sign that he has a problem.