Decide firmly that you will not gamble and then block the gambling sites. Try spending more time with family and friends. Use less internet and try to keep yourself busy with other thing like some new hobby or keeping yourself busy with work. Leaving anything's addiction is possible if you firmly decide to do so.
But if he can block the gambling sites then he can again unblock them whenever he need it if he is addictive, the best solution is that he should not have to sit idle as idleness is the devil which make us to start any bad job.
+1. Our nature is to find another way if we are prohibited to do something we really do want to do. In this example, gambling. Even if you block the gambling websites that you used to play on, you will eventually unblock it especially if you get the urge to gamble. You were the one who blocked it, of course you can unblock it.
And even if you weren't the one who blocked it (i.e. you don't know how to block a site), it's either you will search for a new gambling site to bet on or you will search on Google how to unblock a website.
Get yourself really really busy so that you will not get the urge to gamble. If your mind is so busy doing other things and you don't have time to gamble, it will at least lessen the chance that you gamble even if you get the urge to. At the end of a long busy day, you will be too tired to gamble.
I suggest that you try to focus and be busy on things that doesn't include money like joining a group that does synchronize activities, do exercise, get more focused on your job/career, find a new hobby, and/or study something that you were always curious with.