1. Definitely turn him back around while you figure this out!
2. I would see if something's up with the seat. I have very tall, long-torsoed children and they weren't that torso tall so young.
3. The overall height limit Britax gives is entirely irrelevant. My oldest outgrew the harness on her radian right around her 4th birthday and my 3 year old has less than a year left until hitting the top. Diono does allow the harness to be used when shoulders are above the top slots, but that's not something I'm comfortable with personally. The radian top harness slot is around 17".
4. Sit him down and get his torso height. To do this he needs to sit criss cross against a wall then you mark where his shoulders are and measure. That will give you an idea of how much time you have left.
Hopefully you can get him to age 2, when you can move him to a frontier (there's a minimum age of 2 to use that seat). At that point I would highly recommend that seat if in fact his torso is that long because it has the tallest top harness slots of any seat on the market.