The original author of Don Jr's words has made a statement that he worked on the speech and used his own words. Very probably true, but not necessarily. He's involved with them.
Btw, Jr's apparently a genuine strong conservative and not at all above enthusiastically lying about and smearing Hillary Clinton, which is only one indicator of poor character.
After only one speech Jr's being touted as a "rising star." Shooting star and hubris are what come to my mind. One of two things has to happen: Trump has flown too high. Either he will manage a cushioned fall by merely losing the election, ending up mud in the eyes of a right that even at the convention hates Hillary far more than it wants him, or he'll fall hard and take his disgraced family with him. If Don Jr's so smart, he should have realized that presidential glory was not in the Trump family future and done a better job of protecting them.
Btw, Trump's first of three civil trials related to Trump U begins November 28. AFTER election day.