An inferior good is a type of welfare whose demand decreases when consumer income increases or demand increases when consumer income decreases. Therefore, if a consumer considers shirts to be inferior goods, the way he will stop consuming it will be when there is a real increase in his income.
In the case narrated, Alex had an increase in salary and remained working for the same number of hours. This means that with the same job, he will have a higher income, meaning there was a real increase in Alex's income. If he considers the $ 3 shirts a much lower asset, he will lessen the demand for it.