How long is his torso and overall height? If he's just outgrown/outgrowing a Scenera then he *may* have enough time in one of the combination seats I mentioned to get him to a good booster age (minimum 4 years and 40lbs).
If he does have a really long torso, then the original plan of using the Radian and passing the Scenera to your brother might be a better idea.
Aiden is 5.5 years old. He is 35lbs, 44in tall with a 16in torso. His torso height has not changed since January but he's gotten taller, so it looks like his legs are finally catching up.
http://farm1.static.flickr.com/125/413192536_e6bcc9cb12.jpg?v=0
If the Marathon/Chase are in super upright positions (3rd row of my van), he is just below to right at the top slots on these seats... but he sits in the captain's chair of my van and is over the top slots when these seats are installed in this position. So he has a Radian... he's still got approximately 1.5 - 2 inches before he's even with the top slots on this seat.
If you have either of the combinations seats I mentioned near you and you can try him in them with the seats installed in your car, that would be the best idea. If you do get a chance to do this, check out where his shoulders are in relation to which slot he'd need to be using (example: shoulders are one inch below the second from top slot). That info would give us an idea of how long he might have in the seat. Keep in mind, it would just be an educated guess... nothing is set in stone.