Is it permissible for the the son to benefit from the entire amount, or must he dispose of the amount exceeding what his father paid and the grant, given that it may include usurious money?

The amount a child receives upon reaching eighteen years of age, which the father paid during that period, is halal. As for any increase, if it resulted from a legitimate investment, it is halal. However, if it resulted from an unlawful investment or a commercial insurance contract, it is not permissible for him, and he must dispose of it by spending it on charitable causes, unless he is in need, in which case he may benefit from it. This is based on what Al-Nawawi reported from Al-Ghazali: "He may give it in charity to himself and his dependents if he is poor, because if his dependents are poor, the description [of poverty] applies to them; rather, they are the first to whom charity should be given. And he may take from it what suffices his need, because he too is poor."

As for a grant, if it is given by the state or an insurance company, he may benefit from it if he meets the conditions for its entitlement. However, if the grant is obligatorily paid by the insurance company under a commercial insurance contract, it is not a true grant, and its ruling is the same as that of unlawful insurance money.