I'm Doug, and I'm very sorry to hear of your son's situation. My goal is to provide you with excellent service today.
While it certainly is not very fair. there is no law preventing an employer from taking an employee off shift until they pass a drug test if they have been accused of using drugs while either working, or on a break from work, or if the employee is suspected of using drugs before coming into work such that they may be under the influence when they arrive at work.
Unfortunately, your son will just have to take the test and when it comes back negative, he can return to work. While not the law, many employers will pay back pay for time lost while waiting for a drug test if the employee tests negative, so your son should absolutely ask to be paid for the lost time.
You may reply back to me using the Reply link and I will be happy to continue to assist you until I am able to address your concerns, to your satisfaction.
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I wish you and yours the best this holiday season,