My 6.5y.o. son is 70lbs and 52 inches so he is about to outgrow his Britax Regent since the height limit is 53" and he grows like a weed!

So basically the question is "what's next?" Ideally I'd like to keep him in a 5pt harness as long as possible. (I'm getting the feeling that may be unrealistic much longer, everyone else I know has had their kids in backless boosters for 3+ years lol) The seat will stay in the car so I don't care how heavy it is or how wide/large. Safety is the priority.
thank you