I think it roots in the beliefs and the value that the father holds. Mostly held under their culture or their religion. It's a hard decision to make for the kid I guess. Is gambling a bad thing to do? I don't know. You don't need to know to. What you just need is to ask yourself truly if it is the right thing to do. Because we all have different beliefs. and my belief has nothing to do with what you are believing. Same goes to the story of the boy. It depends on what is more important for him. Is it the culture of his father or is it the money making he can make gambling.
I don't know much about beliefs and customs in Nigeria, but I am sure that this is a pretty stupid thing to do (in case the story is not fake). The amount being mentioned here is huge, and it can be life changing. And in today's world money matters more than anything else. If tomorrow some of his family members get hospitalized and requires some expensive medical procedure, his beliefs are not going to help. I just hope that the son is not as stupid as his father and he will claim the reward and spend it wisely. If his father decides to kick him out of the family, then it is his problem.
Well, I pretty much agree with the logic here, if the family isn't so well-to-do and doesn't have a lot of money, rejecting such an amount is sheer stupidity, because as you said, beliefs or culture or any other thing won't pay the bills and money is a basic requirement for today's world, you can't do anything at all without it, and if wealth is knocking on your door, you should welcome it with open hands.
Such a thing can be understood if the father is already rich and doesn't need money, he is feeding his family well, all their expenses are covered and they even have savings for bad times which can come all of a sudden, but if this is not the case, he shouldn't have rejected the money.