You're right - he's way too young for a belt-positioning booster. You're looking for what's called a "Combination" seat - a forward facing seat that can be used as a 5 point harness seat and later as a belt-positioning booster when the child outgrows the harness. Good ones in your budget range would be the Evenflo Maestro ( about $80, 50 lb. harnessed limit, positions the belt well as a booster but doesn't adjust very tall so you'd need to get another booster after he outgrows it as a booster - this is not a huge deal because a good high back booster like a Graco TurboBooster high back can be found for around $35 on sale. details here http://carseatblog.com/5923/evenflo-maestro-as-booster-re...) and, if you could go a little higher, the Graco Nautilus (65 lb. harnessed weight limit, can also be used as a high back or backless booster - around $160 but you could probably get a good online deal for less than that, and it could actually last until your child is ready to use the seat belt without a booster. Also if your child is almost 40 lbs. at age 2.5, you might need the 65 lb. harnessed weight limit to make sure you can keep him in a 5-pt until he's mature enough to sit properly in a booster)
Most other combination seats in your price range (Evenflo Chase, various models under the Cosco/Summit/Eddie Bauer name) are not worth buying because they either have a low harnessed height or weight limit, don't position the seat belt well as a booster, or both. examples here http://carseatblog.com/1646/dorel-summit-as-booster-part-... , http://carseatblog.com/1426/dorel-vantage-point-as-booste...
PS - when you compare seats, make sure to check what the *harnessed* weight limit is - many are rather vaguely labeled so a box that says something like "for children 20-100 lbs." might have a seat with a 100 lb. booster weight limit but only a 40 lb. weight limit for the 5-pt. harness. Also please note that many harnessed seats are outgrown by *height* before the weight limit is reached (harness slots should be at or slightly above the shoulders for a forward facing seat)