Hi Kim,
I'm a Child Passenger Safety Technician. Hopefully I can help you with this problem (and it's common one!)
The first thing you should check is the tightness of the harness straps. I'm assuming he's forward facing? If he is, the harness should be AT or ABOVE his shoulders. Secondly, make sure the harness is properly tightened. It should be so tight that you can't pinch any slack. The chest clip should be at nipple level and if you try to pinch the harness above the chest clip, your fingers should just slide right off of it.
Once you've made sure the harness is properly adjusted, you can make a little deterrent on the chest clip itself. Get some sticky velcro and stick the prickly side pointing out of the chest clip. (not in towards the child!!) It can be uncomfortable to press on esp with little fingers and that's usually enough to get kids to stop messing with it.
Finally, it's discipline/positive reinforecement issue. If you catch him doing it mid-drive, each and every time, pull over and tell him very sternly that it's not safe and use whatever parenting euphemism you prefer for a behavior that's not allowed in your family. (Examples include, "That's very naughty" "We don't allow that" "that's a no-no" etc)
Hopefully he'll get tired of the trick soon and move on to a new way to aggravate you! LOL!
:-)