Explain Noncompetition Clause and Public Policy

A man entered into a agreement to buy a small business from the entrepreneur who had started it. Formerly completing the formalities the buyer learned that the entrepreneur had taken a job with a larger rival company. The buyer now qualms that the seller is taking vital business information with him to the rival business as well as that this will affect the prospects of the small business. How can the buyer defend his interests? What authorised protection does he have? Can the agreement be declared illegal?