What's his motivation for actually doing it other than not being in trouble?

If there are only negative consequences to everything, then it doesn't matter to do it. Take school work for example. I mean seriously, how smart are Snooky, Honey BooBoo and Kim Kardashian really? We inundate our kids with the idea they could be well off by working hard, but the proof they see is that you either have to be an athlete or idiot to make money (and that's really not far from the truth).

What are the positive consequences to him doing his chores? It's the same type of thing. If the only benefit is not getting in trouble then what does it matter?

Find his currency. Maybe it really is currency (money) or maybe it's praise or something like that. Give him a reason to want to do good in school or at home and he'll motivate himself. It's not easy and a perpetually moving target, but finding his currency is one of the best motivators out there.