Well, its the son's money and he can do anything he like with it. He offered it to his dad, and his dad rejected it. That's it. There is nothing more he can do about it. Maybe his dad is extremely religious or perhaps, gambling is seen very negatively where he lives, hence his dad doesn't want to be seen as a "bad" person or something. The kid can do good with the money. Perhaps start something for his own, spread and donate every now and then. But he shouldn't regret anything. As long as he isn't gambling and affecting his life or anyone, it should be fine. His dad will probably eventually understand.
A lot of things are not clear in this story. What happened to the money? Did the father took that money and then donated it to a church or a charity organization? In that case we can't say that he rejected the winnings. We can only say that he accepted the money and then donated it to someone else. Only in case the father returned all the money to the casino, we can say that he rejected the prize. And I don't think that it happened either. And the casino hasn't made any announcement on this matter and legal situation is also a bit fluid (all forms of gambling are illegal in Nigeria).
The son won a large amount of money and he decide to gave a part to the father, you know how fathers usually are, I mean the responsible ones, they will ask where the son get the money from.
I don't think the father took the money and donate to someone else, there is nothing in that story that sounds like this, I believe the father rejected the money and the son too, weird? Well it has happened around me too, someone living very close.
Before the son won this large amount, I believe the father must have been talking against gambling to the son, he must have severally warn him to stay away from gambling because his religion doesn't allow it, but the son thought that he could confuse his father with the money.