Is a son considered disobedient to his father if he refuses to work with him in his grandfather’s shop because he does not receive a wage, knowing that his uncles mistreat him and his mother? And what is the ruling on refusing to work in this case?

You are not obligated to work in your father's shop if it causes you harm, such as working without pay, and you will not be undutiful by refusing. Try to reach an understanding with your father to pay you a wage. If your uncles speak ill, advise them gently or seek assistance from someone who can guide them and remind them of the sanctity of not harming a Muslim.