Supervisor Charging Me for Use of Vehicle After Being Laid Off

My question involves labor and employment law for the state of: Massachusetts

My former boss granted me a $5,000 bonus for passing the FINRA Series 6 and 63 exams. We used that $5,000 to put as a down payment on a car for me. My credit was not good at the time so the loan for the vehicle as well as the registration of the vehicle were in my bosses name. I was the co-signer on the loan. I paid the monthly loan installments via payroll deductions every month up until I was laid off. I still had use of the vehicle for 14 months after being laid off, although my boss was making the loan payments.
I was having a relationship with him and was living at his home until the relationship ended badly. While living at his home, my boss had full access and use of the vehicle whenever he wanted.

When the relationship ended I signed an agreement (note) stating that I would pay my boss back for the 14 months of loan payments he made while I was unemployed and had use of the vehicle. (somewhere in the amount of $5,000)

I signed the note under duress - the relationship was ending badly and he intimitated me. I actually made 7 payments to him according to the note he wrote up. Then I started to think......I don't think I am legally responsible for these payments, the loan and the vehicle were in his name. So I stopped making payments.
He sold the vehicle after I paid for 4 new tires and new brakes. He paid off the loan and made a profit of $5,000.
There was no lease agreement.

My question is: Am I legally responsible to make these payments to him? Can he take me to court?

Hold the agreement you signed up close to the monitor so we can see it.

Apparently, the fringe benefit package and employment security were not what you thought. If you signed an agreement and it meets all the criteria for a contract in your state, you can be sued for the balance.