Is employment agreement and bong legal and what is its validity

Dear sir My father has been into some legal issue with his ex-company and therefore I request a little help of yours. I hope you consider my queries! Following are some my queries 1-- If an employment agreement valid if its not on a bond/stamp paper and not even signed by the employer? 2-is an employment bond done after joining the company (in the middle of employment)? 3--can a bond restrain me from doing a job/business related to the field my father has an agreement of restrictive covenant?? Also an any further add on to the matter will be of great help! Eagerly waiting for reply Thanks