You can try sticking a prickly bit of velcro on the buckle. It makes it hurt their fingers so they are less likely to try. Unfortunately the stiffness of the button is literally set in the legislation, so changing to another seat is unlikely to help very much unless it's a totally different type as said.
If he is usually amenable to rules, you could try telling him wow he is very clever/big/grown up to have figured it out, BUT now that he is so grown up it's also important to be responsible and safe. When we are driving, we need to keep the seatbelt on at all times, even when the car is stopped at a traffic light etc.
Tell him that he can open the car seat harness himself, but ONLY if he waits for the special magic word at the end of the car journey (e.g. 1...2...3...open! - doesn't matter what it is.) If he doesn't wait, then he won't be allowed to open it, and you'll have to open it for him (said in a stern/warning voice).
Most 2yos are not quite advanced enough to recognise the obvious logical problem with this - the promise of being allowed to do it themselves if they are very good and wait for the word is often enticing enough and they often take the responsibility very seriously. You can also get them to be the safety seatbelt officer, and check that everyone has their seatbelt on (including adults/baby siblings) before you start to drive and at random points during the drive.
The newer versions of the Cybex seats use a sort of seatbelt type buckle on the shield, which can probably be reached by the child, so I'm not convinced they are that great for preventing escapes for this reason. The older style ones, where the shield was held in by the car's seatbelt, were v good as younger children physically could not bend their arms around the shield to reach. You can still buy these, so might be an option if you get really stuck. The Cybex ones convert to very good high back boosters. Joie also have a couple if they still make them.