My 11yr old son has ADHD (diagnosed in 3rd grade). Does he qualify for Supplemental Security Income benefits based on this disability?

Supplemental Security Income (SSI) is a needs-based benefit for adults and children of adults who have not paid into Social Security enough to qualify for Social Security Disability Income (SSDI) and have a disability (a) leading to death or (b) causing them to be unable to hold a job, any job in the national economy, for at least 12 months. From your description, it is difficult to even guess whether the Social Security Administration would find your son disabled or whether your income and assets are low enough for him to receive SSI, a needs-based benefit. Your son might qualify for SSDI, which does not consider income and assets, based on a parent's income if that parent is disabled and your son is also found to be disabled (for children functional ability is based on measures other than ability to work).  You can apply online at www.ssa.gov.  The key is being polite, prompt and persistent -- and showing medical evidence indicating lack of residual functional ability.