If he is up to the age it's better to have some discussion with him, maybe he will listen, if you discipline him in another manner he will probably start fearing you and start hiding things from you, better start with talking to him and make him realize the dangers that awaits every gamblers.
Educate them, teach them what you know about gambling and make sure they get used to risking very small amount of money, and the last part is to always keep an eye on such child, know the type of friends they keep, because it's possible that they get to start gambling through the friends they are keeping.
When you are parenting a child, you are supposed to teach them what's good and what's bad, this includes good and bad manners and everything else, and if you fail in doing that, and then you find your children doing wrong things, you are supposed to blame yourself and not the children because it was your responsibility to show them the right path.