I'm afraid that the answer is no.  The money SSDI gives your son is for his support.  Once he reaches 18 (or 19 if still in high school) SS deems that support to be no longer necessary.  You will not get an amount to make up for this.  You do however get annual increases for cost-of-living on the money you receive. Possibly you can qualify for SSI (Supplemental Security Income).  The SSI program makes payments to people with low income who are age 65 or older or are blind or have a disability.  To find out if you are eligible, contact your nearest SS office.