Answer the question briefly:

What did Putnam mean by Freddie being an exceptional child?

By calling him an exceptional child, Mr. Putnam meant that his son Freddie was specially-abled, called retarded in colloquial terms. He must have been suffering from some mental anomalies, which is why he was not like other children his age, and was slow to learn or read and write. However, using the term ‘retarded’ for his son had not proved effective earlier, because teachers did not express interest in such ads. This is why Mr. Putnam calls him an exceptional child.