Re. the cough, I'd go to the doctor for that one. May be allergies which could be making him feel miserable.
Check out your local library for Love and Logic parenting books. They focus on choice driven discipline, the idea being that kids at this age have VERY little control over their world which is frustrating for them. You want to give them as much control as you can over things you really don't care about so that they learn to listen and make the choices YOU want when it DOES matter. It also teaches them to start making decisions at an earlier age and to start thinking about consequences. Example - my son is pretty good with diaper changing, but sometimes he doesn't want me too. I let him chose WHERE it will be changed - changing table or floor? Either way, I get what I want done, and HE feels he "controlled" the situation. I'm not sure what you mean about sitting in his car seat - can he unbuckle it? Or is he just not happy there? If he's just not happy, I don't know what to say, but if he's unbuckling, you need to get a seat he can't get out of.
Love and Logic is also on the internet, and if you call them, they can send you a list of people who do classes in the area. Good luck!