If you are younger than full retirement age and make more than the yearly earnings limit, Social Security will reduce your benefit. The yearly earnings limit for 2016 is $15,720. If your son were to earn more than $15,720, Social Security would deduct $1 from his benefit payments for every $2 he earns above the annual limit.
For more information on how work affects Social Security benefits, go here: http://www.elderlawanswers.com/how-does-work-affect-your-social-security-payments--7154