How can we help this boy get a better life?

Several years ago, I became the foster parent of a 4-year-old boy who lived with our family for a number of years and who we loved very much. His greatest problem at 4 was his ability to control his anger and would have tantrums that seemed to come from nowhere. At age 10 months, he had been severely beaten, having been hospitalized for quite sometime and I was told he nearly died. After returning to the custody of his grandmother, we stayed in contact with for quite sometime. Having told grandma he was welcome to come to our house anytime for as long as he wanted, she would periodically drop him off to us and he would stay for weeks at a time. He was like one of our own. Somewhere over time, we lost contact with them and tried finding him over the years. I finally was able to do that on Facebook about a week ago. He is now age 22. I find him living in the basement of the parents of an ex-girlfriend (she no longer lives there). He tells me he spent most of the past few years up to age 18 in and out of corrective homes for boys and lived at home with his grandmother only sporadically. He tells me he does not work because it will jeopardize the medicare and medicaid he is on. He mentions that he was determined to be "partially mentally retarded" and not competent to manage his own life. He tells me he never sees any money and has asked to see legal papers regarding himself and grandma, who is living with a boyfriend elsewhere, says she has to figure out where she put them. His explanation any money received from SSI is that "grandma keeps 200.00 and gives his aunt (who lives elsewhere with her boyfriend) 400.00. He tells me that he calls Aunt and asks her to bring food to where he lives and she has done that. I spoke some with the father of his ex-girlfriend to get his perspective of what is happening and he tells me how he is in disbelief at how they do not bother with him and give him nothing.