I make it a rule to (1. Never borrow money from a friend, (2. Never lend money to a friend, and (3. Never sell a vehicle to a friend.
I've learned the hard way, over the years.
Friendships can be tricky, and selling things to friends, can sometimes be even trickier. Many times friends feel that they can take liberties that they would not take with strangers, or if something goes wrong, (like the son pounds the engine and it blows up, then tells his father that he doesn't know what happened), it can cause hard feeling and end the friendship.
First of all, just tell him that you don't want to sell the car, but if you decide to, you will give him first dibs. Don't let them guilt you into something you really don't want to do. I think you are better off to do as you suggest, and offer to help him find another vehicle.
If you do want to sell it to him, spend $150-$200 and get it officially appraised in order to get a true picture of its value. It wouldn't hurt, and it's a deal for you because now you know what your car is truely worth. Lets say it comes out to $15k. Then, you can show him the appraisal, and tell him that you would be willing to make him a deal on it, like say $13k (if that is acceptable to you), in order to be fair to both of you.
Sometimes, you just have to say, "Look, I can give you a break, but this is the best I can do. Period." They should not expect you to take a loss, any more than they would expect that you would charge them more than anyone else.