Disabled adult dependent

My sister claims her son as a disabled adult dependent.  He is on Social Security and pays more than 50% of his support. TurboTax has him listed as "Nondependent-for EIC/dependent care only".  However, my sister’s third stimulus check did not include the 1,400 for her son.  He did not receive a stimulus check for the 1st or 2nd round. Does the IRS tax code recognize him as a dependent?  I cannot find any information on how the IRS recognizes the above category.  I suspect that he is a nondependent and can apply for the stimulus money for himself.