L.P.
Hi J.,
Aren't these kids clever, it'd be funny if this kind of stuff didn't drive us crazy!
Anyhow, I think you need to plan for time out punishments on the road. For instance warn him ahead of time that he will be put in time out in the car (or facing a corner - if your at the mall and the car is too far away)if he misbehaves. Then I think you purposefully go on some errands with the hidden agenda to catch him acting up so that you can try out this new routine. By doing this you'll be less stressed about having to get the errands done and focus on teaching your son the consequences for bad behavior. Then stick to your guns and just like at home, the second he starts fussing with you, get out of where you are and sit him in the car (or a facing a corner). It shouldn't take long for your little guy to learn that you mean business.
I hope this helps! It sounds like you are already doing a great job with discipline though - he's just figured out the loophole!
Take care.
L.