Pidge ... a driving evaluation would be a good idea, even though you want your son to be independent as long as possible. The doctor may be being overly cautious or he may be thinking of factors that you haven't taken into consideration. If your son is having weakness and rigidity in his legs, that could certainly affect his ability to step on the brakes or get his foot off the accelerator quickly in an emergency.
The evaluation would give you and your son and his doctor the objective evidence to discuss this in terms of providing the maximum safety for your son, which I'm sure is your priority. There may be modifications that can be made to the car to increase his safety and prolong his functioning.
The fact that there are more impaired drivers on the road than your son does not justify risking his safety.
Just my thoughts. I am so sorry your son has been struck with this terrible disease at such a young age. That is tragic.