What can I do if my doctor refuses to fill out my DMV medical assessment?

You need to get a second opinion from another doctor that you are fit to drive.

What was the reason for your having to get the doctor to fill out the assessment? If someone has a seizure, or blacks out, under Michigan law, they cannot drive for 6 months. If that is the case, then perhaps the doctor is concerned that you might have an episode while behind the wheel. You can always get a second opinion. However, I would ask the doctor why he won't fill it out. He certainly should explain why he is taking that position. If he refuses to do that, I would go to another doctor.