If he's a compliant rider and you won't lose your voice telling him to sit right (a lot of people call this 'booster training', but since I've had kids too big for boosters for years and they still need reminders to sit right, it's truly a lifelong endeavor, lol), then he's a fine age for a highback now, no need for a bigger harnessed seat.
Some to look at include the...oh, let me just link you here, scroll almost all the way down, lol
http://carseatblog.com/carseatblogco...ded-car-seats/ One that's missing is the Diono Cambria, but it's fairly bulky, so maybe best for an only child or a captain's chair.
If he's not complaining, then you have till his shoulders are at the top harness position to keep using it for now, if he's shoulders creep over the top, then it's outgrown.
Hope that helps!