I'm going to presume that your son was otherwise eligible for FMLA, properly informed his employer that he was taking FMLA, etc. You are correct that upon return from FMLA leave (whether after a block of leave or an instance of intermittent leave), the FMLA requires that the employer return the employee to the same job, or one that is nearly identical (equivalent).
If not returned to the same job, a nearly identical job must:
- offer the same shift or general work schedule, and be at a geographically proximate worksite (i.e., one that does not involve a significant increase in commuting time or distance);
- involve the same or substantially similar duties, responsibilities, and status;
- include the same general level of skill, effort, responsibility and authority;
- offer identical pay, including equivalent premium pay, overtime and bonus opportunities, profit-sharing, or other payments, and any unconditional pay increases that occurred during FMLA leave; and
- offer identical benefits (such as life insurance, health insurance, disability insurance, sick leave, vacation, educational benefits, pensions, etc.).
An employer can't retaliate against an employee for taking leave by firing him or her. An employee on FMLA leave may be terminated, however, if the reason isn't retaliatory. If the employer needs to reduce staff, for example, or because of misconduct or poor work performance, employees who were on leave are not entitled to any greater protection against termination than other staff.
Arguably, while New York is an "at will" employment state -meaning that in the absence of an employment contract or union agreement, an employee may be terminated for any reason at any time, so long as it is not unlawful (e.g., you cannot be fired because of your race or religion)- the timing of your son's termination is highly suspect and the employer will be under pressure to show that it was unrelated to his FMLA leave.
If your son believes he was wrongfully terminated, he may file a complaint with the Wage and Hour Division of the Department of Labor. They will investigate the claim and may attempt to resolve the matter administratively, and in some cases, may even file a lawsuit on behalf of an employee.
An employee also has the right to file a private lawsuit under the FMLA in any Federal or state court of competent jurisdiction. The FMLA is subject to a statute of limitations. This means that, generally, suit must be filed within two years after the last action that the employee believes was in violation of the FMLA, or three years if the violation was willful. (It is up to the court to rule whether the alleged violation occurred and, if so, whether it was willful.) I would highly recommend that your son contact an employment lawyer to discuss what happened if he is thinking about filing a suit.