Non-rural driving… we live in the big city.
I don’t think you told him anything he didn’t already know, so you simply gave him an unsolicited lecture on his faults as a parent. It is not unusual to take unkindly to that, whether the admonishment is deserved or not. If your friendship requires saying something, then your friendship also requires enduring the inevitable cooling off period magnanimously. If you don’t do this often–that is, if this isn’t the straw that broke the camel’s back–he will probably get over it.
You can call him. If he tells you he doesn’t want you meddling, you tell him, “Hey, you heard me say my piece, that was all I wanted, I will stay out of your hair.” If he avoids the subject, you keep your mouth closed with unsolicited advice and demonstrate that you are not going to butt in any more. (OK, you are allowed to ask for warning so you can be off the street when Junior takes his dad for a cruise.

) If he says he doesn’t want to be around you, well, you have a case of someone who lectures and someone who won’t be lectured to, with neither one giving in, so when the relationship came to that testing point, it gave out.
Oh, well. Find friends who don’t mind your admonishments. It will narrow the field a bit, but if you take admonishments aimed at you with grace and give admonishments to others with even more grace, you’ll undoubtedly find a mutual-improvement society happy to have you.
PS If you find the driving lessons have stopped, don’t you** dare **take credit for the decision or say “I told you so.” Just say something like, “well, he will be taking driving lessons before you know it, and I don’t doubt Chad will be an excellent teacher,” or something else you might have said if you thought the lessons were a great idea but found your opinion overruled, instead. You were straying into what was not your business, so once you get out, stay out.