Some activities which come to mind include playing some sport, drawing or even playing video games.
Though, I would be extra careful with video games nowadays, since many of them include mechanics which are very similar to gambling and encourage the player to spend real money in order to partake on them.
Playing video games is not always effective in overcoming gambling addiction depending on individual personality, some adventurous or war video games might help fill the time to avoid gambling activities without buying any items from the game.
For me the main solution to disappearing gambling is that he has committed not to gamble anymore and avoids the gambling environment or keeps his distance from his gambling friends, he has to find a new environment to forget his past and has to have more activities outside to hang out with friends. He must change his mindset to appreciate the income from hard work not to be wasted on gambling, but apply his financial management to family needs and financial needs for your future.
When a person has yet no family of his own, his gambling addiction might just change if he got a wife and kids to feed which he will be responsible enough to take care of and will not gamble anymore. But if the man already has his own family but turns out to be addicted to gambling making the family dysfunctional, it really can be hard to treat for he had already doomed the family.
One way to stop and save these men is to jail them. A considerable amount of jail time makes a person realize their mistake.