I'm not an expert on this, and I would SO hate having to do it myself, but...
I think doing it together is a very good idea. I think, as good as martial arts are overall, an hour a week of core is probably not going to move him forward very quickly (of course, every little bit helps), and you're going to want a few more sessions of something. I've found that gymnastics, especially once they pass an hour and a half a week, has been really good for my kids in terms of core and upper body strength (and flexibility and coordination). But studio time, group or individual, can be expensive. If you want to focus on building up core strength, I would think that the basics are still the way to go--and all those variations of crunches don't cost any money. Maybe you can mix up the specific exercises? Or ask him to be your exercise buddy, to keep -you- motivated, so his goal is to improve your health?
My observation is that kids start to enjoy gross motor play more as they get stronger (it's less struggle, and more fun). And stamina for fine motor seems to go up, too.
