14 month old undoing his carseat harness

Yep, so the title says it all- a mothers nightmare while driving!! My 14 month old has been undoing his carseat harness while i'm driving!! I do make sure it clicks and i pull on the straps to make sure they don't pop out before i even get in the car myself! We have turned him around to forward facing because of this reason when he was just 9 months old!! He does it every now and than so its not an everyday kinda thing, but he still gives me a heart attack when he does it, i have put him on the passenger side of the back seat so i can see him clearly and when he tries i tell him no, and when we get in the car i tell him he needs to put his seat belt and we cant go in the car if we don't have our seat belts on. Can anyone please give me advise on how i can stop this?

Update: i didnt mention that his shoulders are past the shoulder marker so he can turn around, his car seat is the "baby love ezy convertible 0-4 years) australian standard