One of the first things that I would do, since he isn't paying what he is supposed to, is stop paying any of the bills that his name only on them (unless it is a utility bill). And let him know what you are doing.

The second thing I would do is ask that it automatically gets garnished from his wages, and put it into an account that he doesn't have access to. This way he doesn't get to decide how much he pays (well, he doesn't, but until it's garnished, he's going to continue thinking that he does).