I think the Graco Nautilus would work for him. My 7yr old just outgrew the harness on that, and he's 52" tall! It converts to a high back booster to 100lbs, and eventually a no back booster. ITs a GREAt seat for around $150, and your child will be able to expire it easily (car seats expire after 6yrs).
At 2.5, he still very much needs a harness, and he's obviously outgrown the Scenera as his shoulders need to be BELOW the harness slot when forward facing. Get him a new seat ASAP!
Avoid the Cosco High Back booster, the harness height is not much taller than the Scenera so he will outgrow it pretty quickly, and it only harnesses to 40lbs, and he's not ready for the booster yet.
Another one to check out is the Evenflo Generations 65. My friend has two for her 2yr old and 4.5yr old and they are very nice car seats, harness to 65lbs and the nbecome a booster seat. The top harness slot accomidates most kids to age 5/6. its $99, good deal.
The Nautilus will be your best bet though
And just to make it clear, a forward facing car seat is outgrown when one of three things happens; the ears are level with the seat back, the shoulders go over the top harness slot, or the child goes over the weight limit. If even ONE of these things happens, your child has outgrown the seat and it must be replaced at once!