Thank you, I appreciate that! It's the first time I've ever sold a car and just wasn't expecting it at all.Do nothing. Caveat emptor, which basically means buyer beware.
You've not been dishonest about the car. You're not a garage. If he wanted to check it out mechanically before buying, he should have paid a mechanic to give it the once over.
The risk of buying a 2nd hand car, especially one of that age, is that there may we'll be something about to go wrong. It's a lottery. But that's the buyers risk. You have nothing to feel bad about, and under no circumstances offer him any money back!