Is my son allowed to draw off of his mom's SSD even if she gets the maximum benefit?

There are two benefit amounts calculated when a person is entitled to Social Security Disability (SSD) (also known as Social Security Disability Insurance (SSDI) and Title II benefits).  These benefit amounts are calculated based upon figures derived from the disabled person's wages earned during their lifetime.  They are: Primary Insurance Amount (PIA), and Family Maximum. PIA is the amount of benefits that the disabled person will receive each month, subject to cost of living adjustments each year.  Note: these adjustments are not always made.  http://www.ssa.gov/pressoffice/factsheets/colafacts2013.htm  This appears to be what you are referring to as the "maximum benefit." If the child is a minor, and the Family Maximum is greater than the PIA, then the son could receive child's benefits.  For example:  PIA: $1200; Family Maximum: $1500; minor child receives $300 ($1500-41200).  If there are more than one children, then the $300 is divided equally.  Again, these are dependent minor children benefits. If the son in your question is an adult (18-22), and determined to be disabled, then they would be able to draw benefits due to the mother's account without regard to the Family Maximum.