I agree with hr for me that your son needs to contact his employer and determine whether he has been terminated and if so, why.
The federal law, the ADA, prohibits discrimination based on a qualified disability. Whether a medical condition is a disability under the ADA is determined on a case-by-case basis. However, you haven't provided any information suggesting that your son has a qualified disability or his employer was aware of it. Did your son inform his employer that he believed his medical condition was a disability under the ADA and request as an accommodation for it, to take leave or go on short disability? Have other employees taken leaves of absences comparable to his and returned?

