Is the money given by the father to his son to buy a car, when the son was 17 years old, considered permissible (halal) or forbidden (haram)? And must it be returned to the heirs because the other siblings were not given the like, especially considering that the father's intention might have been to buy a car for the family, but he did not explicitly state that?

If the car was bought with your father's money for the family, then it is an inheritance and you are not permitted to take sole possession of it, even if it is registered in your name. However, if your father intended to transfer ownership of the car to you exclusively, without your siblings, then this is permissible if he singled you out for a valid reason, such as your need, or having many dependents, or being engaged in seeking knowledge, or due to the immorality of others. But if there was no reason for this تخصيص (specific allocation), then it is not permissible. However, after your father's death, the gift is affirmed for the donee, and he is not obligated to share it with the rest of the heirs. This is the opinion of most scholars.