Ask HN: How to keep your milestones and co-founders in check. Incentive/Punishment?

One of my co-founder (designer) is a freelancer and he has a bad habit of telling me that the design or asset will be done by tomorrow but that never seems to happen. He is really good at what he does so and he is quite successful as a freelancer although I am trouble getting him to commit a set amount of time to our project. I know he is committed so that is not the issue, its more of a methodical issue then anything else. I want to know what I can do to improve this situation or what I can tell him to do?