Does the owner have to honor my severance agreement made with my boss that is no longer here?

My boss (who hired me, and ran the office I worked in) agreed to a severance package along with some other things I requested when I was hired 5 years ago. All because it was a small company and FMLA/insurance/vacation was not available. The signed agreement was that I would receive 1 week of pay for every year I have worked for the company if I was let go due to lack of work/company closure. Now my boss has moved on to bigger and better things and the owner has decided to shut this office down. I brought up my severance agreement to the owner and he blew it off! He said that he did not agree to it and will not honor it.
Small company, less than 10 employees in this office, less than 15 in the main office. I'm filing for unemployment until I can find something else and need to know if this is worth pursuing (I have to report it in the unemployment paperwork)