I'm pretty sure the minimum height listed for the Turbo with back is 38 inches...but you could check your manual.
Since he weighs so little, you could get a standard combo seat that would keep him harnessed for quite awhile longer. Seats such as the Graco Ultra Cargo, Evenflo Express or Chase, and Century NextStep and Evenflo Comet are all fine choices
http://www.car-safety.org/carseats.html
My DS was 33 pounds and 38 inches at his 3 year check, and has *just* hit 40 pounds now at almost age 6 and 46 inches, and he's just now getting almost too tall for all those seats, so you should get plenty of harnessed time if you buy one of those. I prefer to keep my kids harnessed in our main car...it's safer, they sleep better, and sometimes it's nice they can't reach each other.
HTH,