I
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Well it might not be illegal from your own view and perspective but like zlantann said some religious actually see this act as an ill act and its probably not welcome by them and if the father is a Muslim then I think his reason for rejecting the money justifies his claim but thats if the son is in a stage where he can't decide for himself because possessing such money can even make the son disown the father if its calls for it because money always come with a spirit that changes or amplifies someone character.
I can agree to religion being the reason why the father is against the money and as for the age i doubt it has anything to do with that.
I did a little search on the news and found at that the son is 19 years old which in most case is old enough to make his decision, and I also fount out that the day is not as rich as others as suspected him to be, and according to the image below the family is currently behind two months house rent payment.

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If I'm in such situations, I'd rather disown my dad nor return the money, with the aid of my siblings and mum, I could play that I've returned the money while I hid it somewhere and watch him do his things, soon the young child will be an adult and would need to start a life, then I'll build my world with my money and my dad will have no choice but to accept me and my new world.