If your husband is an employee “at will,” then his employer can direct him to clean the house, mow the lawn or any other legal task he deems to give him to do. Your husband does not have legal leg to stand on by asserting, “You hired me to be an accountant.” It does not matter. His employer can task him to perform any job he chooses.
Of course, in an at-will relationship, your husband is free to refuse to do anything he finds demeaning and to walk out the door. However, the right to quit is largely the extent of his rights in this regard.