J.Z.
When he does this just pick him up and put him in the car. No doubt he is going to scream, but when he calms down explain that this is how it is going to be. Then the next time you are on your way to the playground give him a reminder of what happened last time and that if he does not listen when you tell him to stop then it will happen again. This is pretty age-appropriate stuff, but if you can take care of it before the baby comes, then it will be SOOO much easier to take the two of them out when the time comes. Good luck!