You could put in one of those little black boxes that records driving time and speed. You could sign up your son for a driving course at Bondurant or others. ALL sons will speed, the problem is education. If he has access to a track and proper instruction on how to go fast, he will most likely not speed while by himself.
Wisconsin has a law that disallows a teen from giving other teens rides. The peer pressure is just too much to handle.
Remind your son that the 850 has a fragile tranny. If he puts it into D or R while rolling in the opposite direction - instant death. If he holds the brake while racing the engine (a la dragging) the pressure build up will kill it. On wet roads, the torque from the turbo will cause BOTH front wheels to brake loose, negating any steering effort.
What you need is trust, first and formost. Then have someone teach him how to hanlde the car in the wet or snow. Keep him from driving his buddies around. But, do not kill his desire to learn all he can about cars.

Klaus
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1975 164E running? 1995 854T sweet 1998 V70 R work horse