Having time to order a seat does give you a few more options. Really, it depends on your budget and how long you would like to keep him harnessed. Each of the HWH seats have pros and cons:
You use the seated torso measurement. It will give you an idea of how much longer he'll have in each of these seats.
MA- 17" ff to 65 lb (most straightforward to install, but has the lowest harness.)
Radian65- 17", but you can use it up to 18" ff to 65 lb (both a bit taller and less expensive than the MA, but can be difficult to install.)
Recaro Como- 19" ff to 70 lb (new and still has some issues that need clarification, but most of them are regarding rf installation. It's the most expensive- this week, at least - but has the second highest harness and isn't as wide as the regent.)
Regent- 21" ff to 80 lb (will fit longest, but is really wide and can be difficult to install.)
Apex65 (and clones) - somewhere between 17" and 19". I've seen conflicting data on this one. Most stats cite 17", while real life data suggests it might be taller. ff to 65 lb (the least expensive, but is nearly as wide as the regent and must have headrests behind it).
Was that just more confusing?
Since he already has a 15" torso at 2.5 years, I'd be looking at the Como or Regent.