That little booger! That is a good seat. Is it the belt that he is loosening by lifting the lever by the feet? Our buckles are very strong on our Britax. Have you tried to talk to him and discuss the safety his seats offer?
My daughter just went to swim class for the week. Every day the instructor discuss pool safety with them. She asked do we run by the pool? No. Do we swim alone? No, we swim with the instructor or our parents. Who can show me the lifeguards? (Pointing out all 5).
They really do learn these things, even though they think they are being cute. Just start working on the safety the seat provides him, daily.
It is likely he will unbuckle any other seat as well. Maybe he will be an engineer, already knowing how everything works.