Based on how he is fitting in his current seat, I don't believe we will get much further than 3 rear facing.
I know it is the safest, but I don't think I can afford to buy a radian then have to buy a FR later. Would he still need a FR after he outgrows the Radian?
I have my 2 1/2yo rf in a radian rtx, classic ma, and a nextfit. I also have 5yo twins who I use extensively for carseat testing, both to get an idea of longevity and of comfort.
One of my girls is tall, I'd say 90th percentile, the other is small, I'd say 25th percentile. The taller one still fits but the headwings crowd her shoulders - she would fit better in the 120 I think. Both girls still fit in the nextfit and classic ma, although the classic ma has less shoulder room (one of them might be a smidge too tall for the MA, it's hard to get her to hold still). I plan to move my girls to boosters at age 6.
I think a radian rxt should get *most* kids to an agreeable booster age. It is becoming common to harness longer and longer and the fr90 will certainly let you harness longer than the rxt but that doesn't necc mean the rxt won't get you to booster age. It all depends on how fast your kid grows, their temperment, and when you want to move to a booster.
My girls normally ride in fr85s and I don't see us harnessing to the full ability of that seat. It really comes down to your child once they hit that 5/6 age group. My oldest son outgrew his marathon before age 5 even though I think he and my taller daughter were comparable in height - he must have been more torso. He might have been too tall for the radian by 5 1/2. It is so hard to say what will work and what won't.