Can my stepson chose to live with me

[trevor]

I have brought my stepson up since he was 6 months old. The father is non-existent and isn't mentioned on the birth certificate. I had 2 children with his mother, and separated from her in 2005 with subsequent divorce. Contact was regular until I met a new partner then had to go to court for access rights. At the court, my stepson was included in the contact arrangements due to the time I was with him bringing him up as my own. I still have contact with him. He currently calls me Dad, even though he was told I wasn't his father.
Now, aged 11, he wants to come and live with me and my partner. Is he able to do this legally, and what legal rights would I have?

[trevor]

The childs mother will not be very happy if he does decide to live with me.
I must point out that he is my stepson, although I raised him as my own