Someone should tell that moron that you don't withdraw from NAFTA by issuing an executive order. Did anyone in the White House try to look at the NAFTA treaty? Section 2205 (helpfully titled "Withdrawal") states pretty clearly that you do it by writing a letter to Canada and Mexico:
A Party may withdraw from this Agreement six months after it provides written notice of withdrawal to the other Parties. If a Party withdraws, the Agreement shall remain in force for the remaining Parties.It took me 2 seconds of googling to find the withdrawal language.