Have you tried fluency games on the computer? Some kids find it frustrating but DS loved Big Brainz which has addition through division. Multiplication is called "timez attack". DD8 also struggles with this with her slow processing speed and I just keep trying to plug away at it. I think the key is to have him do it daily, or almost daily, but even then he may still forget some of the facts after a while or be slow to retrieve them. Big Brainz has free versions so you could have him try it and see if he likes it or not. It's a video game format so I think that helped keep my kids engaged. DD went through phases where she was frustrated by it, probably because of the speed/timing aspect of it. If he is very frustrated by being timed (even if it's a game) he probably won't like it.
