son's wishes

For issues specific to autism / Asperger Syndrome.
My 11 year old son has moved in with me. He is very happy with this situation and has no problems voicing his opinion on this. How can I make this arrangement formal? I appreciate that despite my son's wishes and what is in his best interest this is a tall order. Maybe more realistically: how can I get acknowledgement and recognition that I am his full time carer?

Thank you for reading
Does he have a disability?