In stock form, he's going to have to be really good with the clutch and throttle control to ride in those kind of conditions and not hate it. The CR85 is stronger in the mid to upper rpm's, not so much on the bottom to mid. The RM and KX 85's and KX 100 are stronger in the bottom end, but they are still weak compared to a KDX or a CRF 80.
If you don't mind adding a flywheel weight, a pipe, and possibly even porting it, you could make it rip in the woods, but you will get more low-mid power at the expense of high RPM screaming.
He's at a tough age, where the best bikes he fits on, aren't really made for the type of riding he's going to be doing. And the bikes he fits on that are made for the type of riding he does, aren't as high performance. All the 80/85 2 strokes are motocross bikes, and they are meant to go fast on a track (small bore 2 strokes like to rev), which doesn't always make them ideal woods bikes. They can be made into good bikes for the woods, but stock they have some room for improvment.