To me it seems like the age old problem - lack of communication. Why is it that people find it so hard to talk to those that they love, and are closest to? As others have said, talk to your husband about what is going on, how he plans to help his son grow up and become a useful member of society, or alternatively why he is babying him, and allowing him to walk all over you two as a couple. Once this has been clarified, then your husband should talk to his son, and follow through with the plan that you have both agreed.
With regard to the car situation, it would seem that originally you each had a car, and for some reason decided to sell the vehicle that your husband used, and possibly paid for. Having done this ourselves, I still tended to refer to 'my' car for a while, but gradually got out of the habit as time went by, probably, because we both contributed to the running of it, ie., we both put petrol in it, any repairs, together with the tax and insurance, was paid for out of our joint account, so it just became 'the' car, rather than 'my' car. If your husband is contributing in other ways, ie, he pays for a gardener, or cleaner, or for decorating, while your contribution is paying to keep the car on the road, then you have to stop thinking of it as your car, as it's now a shared car, just like your home is a shared building.
Good luck moving forward OP, but do please start communicating with your husband, and step son, as to allow matters to fester, rather than dealing with them, is the highway to nowhere.