Does he lose or gain any room in the car?
It's hard to say for sure from the angles, and I'm not always the best judge, but I'd say he has an inch at most. How long that takes depends on how/how fast he grows, obviously.
I wouldn't bother with an RA55. I don't think that's going to do much for you. You need a tall shell, so you're looking Radian, a misused Complete Air or TrueFit.
Anna just outgrew her Coccoro so I went Complete Air, but we know how that turned out.
I stuck her in the MyRide at Target and was NOT impressed with how little room she has left. But I figure her rate of growth has slowed enough that it'll buy me a little more time until something else (hopefully) comes along.
With your kid being a year younger, though, and already being so tall and probably still growing fast, it probably wouldn't work for you.