I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
Wrongful termination, from a legal perspective, means one of three things: Your son was fired in violation of a contract that says he could not be fired without due cause; he was fired due to unlawful discrimination based on his race, gender, ethnicity, age, religion, or status as a member of another protected minority class; or he was fired for reporting violations of the law to the appropriate authorities. Because employment is at will, no matter how wrong a reason for firing someone seems to you or me from a moral perspective, there's no cause of action unless the termination fits into those categories. There is nothing in the facts you've provided to suggest that any of those things are the reasons for your son's termination.
Employers do not have to pay employees for unused sick leave when they are terminated, unless there is a contract or employee handbook that says so.
Unless he was an exempt employee, he must be paid for all hours worked, even on his days off. Exempt employees are paid on a salaried basis (meaning wages are never deducted if he comes in late or misses a day). However, if he was on the custodial staff, it sounds like he would be an hourly employee, not an exempt employee. He can file a wage claim with the Department of Labor for all the hours he worked that were not paid.
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