H.P.
my idea would be to pull over and not move the car if he unbuckles or even to take him strait home and (if you can) leave him there while you run erronds. tell him the reason he can't go is because he keeps unbuckling his car seat. give him a chance every few days. i'd also give him a reward (just something small when he stays in his car seat. hopefully you can get him to stay soon.