Anything other than "Thanks, Coach" would be ridiculous.
Imagine your father playing for Purdue, you and your brothers growing up cheering for Purdue and then playing for Purdue, and then meeting with success as a coach somewhere else, finding yourself in a position of declining your alma mater a second time or returning to Purdue and being welcomed home as a conquering hero/favorite son.
This is a unique situation and isn't anything other than "Mama Called."