The Radian RXT or R120 would be good for him with the 45lb RFing limit, the Graco Size4Me/MySize/Headwise would be great for height.
Though the Radians would
also be good for height (very tall shell) and the MySize/Headwise for weight (40lb RF limit).
I cannot imagine very many children outgrow the Radians by height, nor the MySize/Size4Me/Headwise by weight, much earlier than age 3.5-4. They are both great for both height and weight.
However, I'm concerned he will outgrow most seats by height before he reaches 80 lbs.
kaismama-- your son most likely
will outgrow harnessed seats prior to hitting 80 pounds, but that's okay as long as he is able to sit in a booster seat properly (which for neurotypical children is
usually around age 5.5-6.5).
By the time your son outgrows whichever convertible you choose now, there will probably be some great combination seat (harness to booster) on the market that will get him to safe boostering age (if he isn't already by the time he outgrows the convertible). There are already some really great combination seats on the market, but in 4-5+ years, who knows what incredible seats might be out!
It is possible that your little guy will slow down significantly in growth and will make it to age 5-6 in the convertible. Sometimes that happens. My niece and nephew were both REALLY long as young babies (parents are 5'11" and well over 6'0"), but slowed down
big time and are now pretty much average. You never know!