If I understand correctly you are traveling with your grandson, not your son who has a DUI. Therefore, your son is not involved. Just get the letter and the letter requirements signed from your daughter-in-law and probably even your son. It's not going to have an effect on your grandson. The letter is just giving you permission to be traveling with your grandson. However, he will have to have correct documentation. Call and talk to someone about the issue. Have a great trip!
https://help.cbp.gov/app/answers/detail/a_id/268/~/children---child-traveling-with-one-parent-or-someone-who-is-not-a-parent-or