My DS is the same as your son, he can open anything. And at about the same age he undid his car seat. I've never been so angry or scared before or since. And it scared him. He never tried it again, well, so far he hasn't. Since then I've talked to him about why has to be in a car seat and now at 3.5 yo, he'll help me do up the buckles and will position the chest clip properly etc.
On a side note, the only child proof locks he can't open are Tot locks. They require a special magnet to open the lock and we keep the magnet on the fridge. It worked until DS pushed the dining room chair over to the fridge, climbed up, took the magnet off the fridge, opened a cabinet, took what he wanted, then returned the magnet to the fridge and the chair to the dining room. To solve this, we now put the magnet in progressively higher positions. Without the magnet, it is impossible to open the cabinet, as a baby sitter discovered one evening - she forgot where the magnet was.