If he's been OK with the process in the past and he hasn't had something happen to change his mind, then it really sounds like a power struggle. Kids can, and do, work on our Mommy feelings to get what they want, sometimes just to see what they can get away with. I would explain that you expect him to be a big boy about it, and just do it. The fact that he's "insisting" you get out and walk him in is a clue. You apparentlydid it already because he insisted, and now he's very impressed with his power and doesn't want to go back to doing it your way. Get in line, get out of the car and get him out of his carseat if necessary, kiss him good-bye, and leave. If you are trying to coax him, or encourage him, or bribe him---stop. Be very calm, but show him this is the way it's going to be. He'll be fine.