D.W.
Britax makes a seat that can keep him in a 5 point harness till he's 85lbs and you can use it as a high back booster up to 120lbs. That should get him thru! Had it been available when I got my son his Britax back in 2008, that's the seat I would have jumped on.
We have 3 Britax seats and I love them!