I would let him get about 3 mistakes high and then have him move the controls to their max. That way he will be able to see that he can control it. I was having the same problem when I was taking flying lessons (full size planes) Very timid on doing the stalls. My instructor ask why I was having so many problems doing the stall when everything else was excellant. I told him I was afraid I was going to spin it. OK, lets take it up and you spin it. Tried it and all I do was get a sprial, he said you can't even do spin when you try, so he took it and did a spin. After that I was not afraid to practice stalls. Just my 2c worth.