someone told me just yesterday that, once they are 12 the child can state who they want to live with. it doesn't mean you won't need a court order to back it up.
I'm assuming you're in the UK and that you want your son with you.
its a difficult one to advise
factors to consider
What the person with custody wants and is happy with
Does your son fully understand what it all means
Boys mature slower han girls so courts often look to a boy to be 13 even 14 before he is allowed to make the descion - sorry if this is not what you wnat to hear
My daughter aged 12 said whe wanted to live with me placed her '4' perents in order of preference with me and my other half in 1st & 2nd followed by step mothere then lastly her father and the court still decieded to leave her with him - also it was to do with a school she didnt want to be at