They have been given a week and they have dodged you 2 times?
Get someone else?
Give them a 3rd time or find another?
2nd time I've gotten a person.
1) Get an estimated timeline up front. Pay 30% of estimated cost at that time.
2) Allow for 50% extra time for the unforeseen shit that comes up (ie a job takes 4 weeks, allow for 6 weeks total no more).
3) Pay 50% of the total job at the halfway timeline mark
4) Pay the other 50% upon completion of the work and make sure it is to your satisfaction.
The reason for the layered payment schedule is if he is using his own money, he has stake in the project to make sure its complete to get full payment. I've had half a dozen projects with different vendors on this. All of them had no problem with this set up.