Well, first you are going to have to get him to sit in his car seat at home on the living room floor. Give him a cookie or special candy or something like that - never mind that it is bribing. Bribery is o.k. if they do what you tell them to do when you 'bribe'. This is the way that we got my gr grandson to like church. He would cry on the way into the church, an Usher would hurry out with a cookie, then another cookie when he sat on my lap in church, then off to his class and he was happy without a cookie. So you see what I want you to do with the treat and him sitting in his car seat on the living room floor. As he sits there read him a book, sing to him, talk lovingly to him, or show a good short kids video that he loves, you sit there with him too. Then, pretend that you two are in the car and go for a short ride, tell him that you are at the preschool or restaurant or whatever and then get him out of the car seat and into the dining room and sit down, pretending you are at the restuarant and so on and so forth. I don't think I know how to spell restuarant.
Also, you are going to have to be more firm in your day to day discipling of him. You need to teach him to listen to your voice and obey, so therefore you do not talk unless he is attending to you. Just talk low and no yelling, and no saying NO just go through the movements of teaching him what to do when you talk to him. You may have to give up and stop once in a while, but eventually it will work , - always remember that his attention span is shorter than you can now imagine. It will get longer and you can lengthen it by doing this sort of thing. Being firm is the only way - not spanking or yelling, but be firm. Say, " I am your mother and you are going to do what I say "- don't say that too much because if you rile him up he will really get tough about changing. Lure him with love and firmness. Tell him that he cannot go to the resturarant unless he learns to sit in his 'restuarant chair' at home and eat like people do. Oh, la, la, la and on and on , you get the drift. Good luck and be strong, C. N.