If, however, you were conversing with said rabbi about getting married to one of his congregants or to his daughter, or about being elected president of the synagogue, and neglected to mention that you are not Jewish, then I can understand his dismay. Presumably your pitch to potential donors is the same whether you are appealing to Jews or non-Jews. The only thing you are guilty of is not disclosing a lack of respect for your donors. The suggestion that people, specifically Jews, will donate only to their fellow Jews is insulting. The cause is the cause.
People donate because they believe.
My 6 year-old son is very close to his grandfather. He lives nearby and picks our son up from school on a regular basis. That was 20 years ago. On the information ladder, I place doctors and their predictions on the bottom wrung. The more shared experiences, the stronger and sweeter will be your son's memories.