I was in similar circumstances when I was 17. My father and stepmother were alcoholics, my mother died when I was 12, I had been in and out of foster care (they just kept sending me back to my dad). I was a straight A student, well behaved and involved in many school activities.
To make a long story short, I came home from a date one night and my stepmother was drunk and kicked me out. I called my boyfriend and his Mom (a teacher) came to pick me up. She immediately called a friend of hers at social services to ask what to do so she wouldn't get in trouble. They gave her temporary custody and started the whole "reunification" process AGAIN. I jumped through all the hoops, paperwork, counseling, etc. Since she was not a foster mom, but someone who genuinely wanted to keep me she went to court and asked for either emancipation or custody. The court said since I was so close to being 18 they felt it would be in my best interest to give them guardianship of me until I was 18. After I turned 18 I lived with them for 2 more years while I worked part time and attended college.
So everything was handled through social services. But the court ordered my Dad to provide copies of his tax returns for financial aid purposes.
I honestly don't know what would've happened to me if she hadn't stepped in. When I was in foster care, I would stay at one home for one to two years, but then they would send me back to my Dad because they were worried about caring for me financially, no one wanted that burden. By the time I was 16 I felt like no one wanted me. It's great that you're willing to do this for him.