After strapping the 2-year-old into the car seat he immediately takes his arms out of the straps. The only way to get them back in is to stop the car and put them back in. Each time after that the arms come out again.
He is still locked in at the hips.
First: make sure you have the belt sufficiently tight. A tight enough 5-point carseat belt should not allow the child to remove his arms. Tight enough means you can get two fingers under it but not more than that. It's probably a lot tighter than you think it should be - and definitely will be tight for him.
Second: talk to your child. Two is old enough to start understanding why. Keep at it. Model for him. Each time you have to fix it, stop like you say, fix it, and talk about why he needs to keep the belt on properly.
I had a similar issue with one of my kids. She would slide the chest clip down until it was low enough on her torso to get her shoulder free, then she would climb out of her seat through the top of the straps. I solved this by replacing the chest clip with a third party clip that included a strap that goes behind the child's neck, so they cannot slide the clip down.
The one I used is available at escapeproof.net, along with a cover to keep the child from pressing the release button.
