Hi OP, I understand your frustration-but I think it comes more from the letting so many other things 'slide' than it is about his wanting some notice of your arrival. Perhaps you can simply change the rules. I don't know how much of this is done with lawyers, and how much you have just worked out for yourselves. If you think he is taking advantage in the situation-housing, utilities, child, etc., then decide what battles are really worth fighting. I know that in divorce there are myriads of them. If the house is for sale, do you have anything in writing about the condition the house must be kept in? Do you have an agreement about what kind of offer you are both willing to accept? It is costing you money every month he is there, and he is getting a really good deal-which is not in his interest to forgo by actually getting it sold. So make sure your financial butt is protected from his agenda.
Perhaps he could agree to keep several changes of your son's clothes in the garage, so that you can retrieve them when necessary. I don't think calling before you are coming over is that unreasonable, but if you are just accessing a garage, it should only be informational, not in the way of asking permission, and not off limits if he doesn't answer the phone.
Decide what issues you have doormatted on and see what you can do, if anything to change them if possible. If nothing, accept it and make note for the future. Good luck!