Actually, he can run most any track in third and fourth. But as a beginning 65 rider, he needs to learn to downshift. I used to stand on the outside of a corner and point down with one or two fingers while mine was coming into it, that was my signal for downshift and the number of fingers told him how many times to shift. It helped a bit. Other than that, explain how to listen to that motor.
You could also find a nice long uphill and put him back on the 70. Make sure that the hill is long enough the he will lose momentum in third and have to downshift to keep it moving (a dirt road running up a mountain would be perfect). That will help drive home the advantage of downshifting.