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.