I guess I think a person really only NEEDS one car. If they have extra money and can afford a separate car for fun, that's great, but I would never expect someone to provide either vehicle for me.
Before buying a car, I take into account what I need it to do and I purchase a vehicle that fits those priorities. First what I need, then what I want.
If your husband was fine with it, then that's that, but I would tend to think the son maybe isn't making the most mature decisions if he is choosing to use his car resources to buy what he wants and relying on his dad to give him what he needs.