To be safe, please consult a labor law attorney (one who represents employers) to advise you. You don't want to do anything that will prompt a law suit - especially in Federal Court.
As previously mentioned, you should hire a labor attorney to ensure that no future issues arise.
I would like to comment that if your employees have been working for you and you are simply asking them to sign a new agreement that includes new terms that benefit you/your company but they are not offering anything new/additional in exchange, then the agreements could be found to be void for lack of consideration (bargained for exchange).
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You cannot make his continued employment conditional on his signing the agreement. What I would suggest is to offer him new consideration (meaning, offer something, such as a benefit, day off, etc, in exchange for his signing). You will also want to make sure that your new agreement is reasonable in both time (number of years it restricts) and scope (location that it restricts). If you don't have his original signature it will be very difficult to enforce, and even if you did, a 4 line agreement is likely not strongly worded enough.