The "headrest rule" with the Apex is that when seated in the Apex, the midpoint of child's head must be at or below the VEHICLE's seatback. Basically, the didn't design their headrest to be structural, so the vehicle needs to provide support behind the child's head as the Apex can't.
So, unless you have a bench seat or a just a low seat, it should work. Put the child on the seat directly, how far does he have until the tips of his ears are AT the car's seatback or headrest? Assume the Apex would put him 4 inches or so up in the air.....
At five years old nearly 40 lbs, there isn't much for long-lasting options that are less expensive than the Regent. Apex, Safeguard Seat (very $$), Safeguard GO (probably too short, needs tether, easier with lower anchors), the big Britaxes and Radian (probably close to being too short). Apex and Radian are really your best options.
Is he too wiggly for the booster?