That's why I think it's not that he can't hold it, but that he is doing it for attention. When he makes a mess, don't you stop what you're doing and immediately focus you attention on him? Sounds like he is well trained, just not to do what you want!

Don't feel bad about crating him more than usual for a couple of weeks. It's a small amount of time and in the long run it's going to make your relationship a lot better. You've got to take some fairly extreme measures to show him who is boss and that he has to be the one to do what you want.