You can look at growth charts to see what percentile he is in, and if he has a trend of staying consistent in height, at least, to figure out when he might be able to go without a booster.
For what it's worth, kids should be in boosters until they can pass the 5 step test, found here:
http://www.carseat.org/Boosters/630.htm (it's about much more than just age, height or weight).
The 5-Step Test.
1. Does the child sit all the way back against the auto seat?
2. Do the child's knees bend comfortably at the edge of the auto seat?
3. Does the belt cross the shoulder between the neck and arm?
4. Is the lap belt as low as possible, touching the thighs?
5. Can the child stay seated like this for the whole trip?
Highback boosters provide some side impact protection as well, so even if a child may pass the 5 step test, it wouldn't hurt to keep them in a highback booster if they still fit by height (and my 5-foot-tall 12yo little sister still fits the Britax Parkway booster).
Really, if you were looking to spend $180-$200 on a Radian for dd and $200-$250 on a Husky/Regent for ds, spending an extra $50-$70 on the Young Sport shouldn't be a big deal, especially since you'll have an awesome highback booster out of the deal...you wouldn't have that with the Radian.
Normally I'm all for higher weight limit seats, but you'll already have the Husky/Regent. Your ds will only realistically use that for 2-3 years, and the seat has a six year lifespan. It makes sense to pass that seat down to dd to maximize its useful life, and if you had a Radian for her, it's unlikely that she would have grown enough in 2-3 years to benefit from the higher weight limit and taller top slots.
I hope this makes sense. I hurt my back today and took a muscle relaxer and painkiller, so I may be a bit loopy...hard to judge myself, LOL.