Is it permissible to search for another job despite the man's help and good treatment, and after unfulfilled promises to work as a driver?

The employer is obligated to fulfill the agreed-upon contract. He is not permitted to change the type of work agreed upon unless by mutual consent, or if the contract explicitly grants him the right to make such changes. If it was merely a promise to employ him as a driver, then he should fulfill his promise out of religious obligation, even if he is not legally bound to do so. As for the worker, he may accept working in the kitchen, or he may seek another job if he is not content with remaining in this profession, because the employer did not fulfill his promise.