did your son do the proper things before he left. Like give written notification to his employer? Has it been about 6 months or more than 6 months? 180 days or less of active duty they have to rehire within 2 weeks. If longer it gets more vague. I know re-employment after a full AD term can take up to 3 months before you can start really firing alarms.
The DOL is who you file complaints through. Here are some links:
Compliance Assistance By Law - The Uniformed Services Employment and Reemployment Rights Act (USERRA) (http://www.dol.gov/compliance/laws/comp-userra.htm - broken link)
USERRA (http://www.esgr.org/site/USERRA.aspx - broken link)
Uniformed Services Employment and Reemployment Rights Act