I worked as a Claims Rep doing SSI for the Social Security Administration for 3 1/2 years. Many parents do file for SSI for their kids when the child has ADHD. Let me just give you an overview of SSI:
There are two requirements for SSI - it's a needs based program so you have to meet 1) income and resource requirements, AND 2) a disability requirement.
To even be able to apply for your son you and your son both have to meet income and resource requirements. The Rep will ask you about your living arrangements (where you live, who lives with you, etc), all of your income (earned and unearned, like child support), all of the things you own (bank accounts, automobiles, life insurance, etc.) and from there will determine if you & your son meet the first half of the requirements to be eligible. If you meet income & resource requirements, then the Rep will proceed to the disability portion.
Your son would have to meet disability criteria. For an adult, a disability is: "anything that keeps you from working and is expected to last one year or more". For a child it's a little bit different and there's no good explanation. The disability examiner has to look into how your child functions compared to other children his age, how your child is doing in school, whether the medications are working, etc.
It's certainly worth a try - if your son gets SSI, he will get Medicaid. IF it turns out that your income is too high for your son to be eligible, what you need to do is get an informal denial letter from the rep (the formal one will come in the mail in 2-3 weeks), go to DFCS, and file for Medicaid under the KATIE BECKETT provision. 'Katie Beckett' allows children to file for Medicaid regardless of their parents income...but to do this, you have to have tried SSI first.
Write me if you have any other questions!!! Good luck!