My understanding is that the son should not have told him that the money was the result of gambling, there were many reasons for him to tell the money from other results, of course those who know the character and disposition of their own child's parents, surely he has reasons to refuse the money.
With a very big amount of money for their size and it can change their family's life for the better it will be difficult to hide the fact where the money came from, maybe he can use a fake story but of course it has to make sense because a fake story that is hard to make sense of will the harder it was for his father to accept the money, it could be that his father was a person of high moral standards who would not accept money not only from gambling but also money of unknown origin, so there is no need to force his father to accept the money
The father has the right to refuse the money but has no right to refuse his son using the money for his son's personal life, so the father must let his son use the money and it's up to him what it is used for, and the consequences will be borne by his son, especially since he can be said to be an adult.