You can always make money the issue, even if it's not. In that case, having your son there is a big vote of confidence for the school instead of a case of doubt: "Even though its expensive, we want him with you part of the day."
In the end, you're the customer, and you don't have to fall for the hard sell. You can just smile politely and thank the director and tell him something vague like, "We think this arrangement is best suited for DS's needs." Your child, your money, your choice. If you can get past your need to explain when no explanations are really needed, it might make things easier.
Just a thought...
