Have you thought about going back to skip counting? I made my son, also ADHD and very strong-willed but very smart, some index cards for the 7s, 8s, and 9s. He really struggled with those three the most. I turned the index card vertical and wrote in a large size the first number (e.g. 7, 8 or 9).
Then down the card in two colums I wrote the multiples (e.g. 14, 21, 28 ….) for the 7s for example. I had him carry those around everywhere. He didn’t love it, but I would notice him studying them.
I also highly recommend the Multiplication Wrap-ups. I found them (and the division ones) used. They are so fun, even my little ones play with them.
http://www.amazon.com/10-DAYS-TO-MULTIPLICATION-MASTERY/dp/0943343003/ref=pd_sim_sbs_b_3
**Mine didn’t come with anything but the wrap-up itself but it really works and has through the 12s**
Another possiblility is card games like you play WAR but test his multiplication memory while playing. Instead of the person with the highest card getting the two cards, the person who says the multiple first wins the cards. You would obviously need to allow him some opportunity to have a chance (only you know best on that). I allow 2 seconds before answering with my 10YO son. The nicest thing is you can take out the cards as needed. Take out the face cards if they confuse him. Here is a link to more card game suggestions.
http://letsplaymath.net/2006/12/29/the-game-that-is-worth-1000-worksheets/
I also printed out a really handy free e-book full of math games you play with a deck of cards. If you go to the link below and scroll down to the picture with “Acing Math One Deck at a Time”, then you click on it and it pulls up the PDF you can print out. I liked it so much, I printed it front/back and had it spiral bound.
Try to find ways to make drilling more fun. That is the key I think. HTH