If its nothing you did wrong, tell him this, trust me....
"Sir, when I first detailed your car, what I did was remove years of improper care. I polished the paint and gave you a clean slate. The question is then, why did I need to give you a clean slate..."
Let him ponder for a second...
"Because improper care reaks havok on your cares finish. If you shell out the money to have it detailed but then don't keep up with it, it will obviously resort back to that intial state and need to be detailed. Now I don't want this to happen, and you don't want to this happen so I would be happy to go over some proper techniques you can use to keep your car looking amazing.
I also offer a matience package that will extend the length you need between major detailing tremendously and keep your care looking its best. Its up to you..."
Be exact, put it in his face politely, don't fumble your words, and trust you are right. If you get nervous or back off of what you say he will quickly loose confidence in your diagnosis.