Most backless boosters in the US have minimums of 4 years old and 40 pounds; a few are as low as 30 pounds. However, age and weight ranges are broad for a reason: so the manufacturers make more $$.
The truth is, every step up in child restraints is a step down in safety: from rear-facing to forward-facing, from harnessed seat to highback booster, from highback booster to backless booster, each step demotes your child's overall protection. That's why most of us CPS techs will advocate for extended rear-facing (keeping a child rear-facing to the maximum limits of the convertible seat, which, in the US, is 30, 33 or 35 pounds depending on manufacturer) and extended harnessing (keeping a child in a harnessed seat beyond the minimum 4 years old and 40 pounds). This type of information is not readily available to the masses, mainly because the options are always changing. Ten years ago there were no seats to rear-face beyond 20 pounds in the US, despite the fact that Sweden has rear-faced kids to 55 pounds for years.
Anyway, off my soapbox now!
How tall is your son, and how much does he weigh? Can you get a torso measurement? You can do this by sitting him in a harnessed seat you have now and reporting back where his shoulders are in relation to the top harness slots. Knowing this will help figure out which seats he would have growing room in.
I would at least keep your son harnessed another year or two; longer, if you're open to that. If you'd like to see crash test videos of harnessed vs. booster, let me know.