This is not always the case, although children often copy their parents. If a father can show by his own example that his experience in gambling was bad, that he lost much more than he won, then this can help the child make the right decision, if he sees that his child is disappointed with gambling, that he has spent a lot of time and a lot of money, then perhaps it is the father's example that will be the reason why the son will never start playing.
Besides the family, friends can also have big influence in getting gambling habits. If his friends gamble, person can feel pressure to start gambling in order to get along with his friends. This is specially highlighted in teenage and early 20s when friends have most influence on person.
On the other hand if his friends avoid gambling, then he will probably avoid it too.
Children do not always follow their parents' example. If a father shows that gambling is bad, that he has lost more than he has won, he can influence his son positively, perhaps. But that is not necessarily true.
Friends can have a big impact: if they play, you feel pressure to follow them. But even here, that's not a given.
I think it's a personal thing that matures with many psychological factors, not just one