Have him do a few examples using both methods side by side. Use a blue pencil for one method, a red for the other. You might also take a step back and add 18 cents to 18 cents, and track where all the pennies and dimes go. That EDM method there can be confusing if you don't fully understand why the two methods work. However, that method is a good one to know and understand because it really can make mental math easier.
Do you have a plan to coordinate with the teacher about the lost 30 minutes a day of class and teacher interaction time? It's great you got things figured out before things got dire.
