The reason that more upright is sometimes called safer for heavier children (close to the limit of the seat) is because it reduces downward rotation, at least in theory. One of the tests that rear facing restraints must pass is a downward rotation test. We start out by installing the restraint between 30 and 45 degrees; during the course of the crash test, it is allowed to go no farther than 70 degrees. The idea behind the statement that it's safer to start out more upright makes the assumption that starting out more upright, the seat won't rotate downward as far. It may or may not, depending on your particular vehicle, carseat, and install. At one point, I had the RFing Radian installed more upright. It easily passed the one inch of movement at the belt path test, but when I gently moved the top of the restraint, the bottom of the carseat slipped on the vehicle seat and then it wasn't as tight as it was. In this particular case, I'd be willing to be that a more upright install would rotate as much or more as letting it install where it wanted. It might just be another of the Radian's quirks, but I think it illustrates my point that without testing, we don't know that installing more upright initially will result in less downward rotation.
You can install an infant seat more upright, just like a convertible. It may be more difficult to get a full 30 degrees, but it is permissable.
Since you don't take it out of the car anyway, there's no real good reason to keep him in the KF. It's time to start shopping (even if it's shopping from your garage) for his next seat.
My DD is getting too close to the limits of her seat (3.5 pounds and several inches above her head) for my long-term comfort, so last week I ordered the convertible for that vehicle. I'm perfectly fine with her riding in the infant seat in the meantime, but it's time to get her next seat. That way I don't even have to worry about any sudden growth spurts.