My son (11.5 yrs) has recently expressed strong preference to reside with me (father) and has been now for 2 months. This follows increasing requests to visit more frequently and for longer periods. I have a shared care (no court orders) arrangement with his mother which operated reasonably well until 15 months ago, when the mother announced she was moving 150miles to live with boyfriend and Son has no option but to consent to it and change schools etc.

I reside within his school catchment area and am remarried (3 yrs) to my partner of 10yrs. My ex-wife has increasing caused distress to my son by preventing any flexibility in his requests to increase visits and he has phoned me numerous times to visit when he could get away from his mother which I did not endorse. However prior to accepting he reside with me, he has 'run away' and turned up on my doorstep (once 6.30AM) without any knowledge of myself or his mother. As he is clearly at risk of running away and determining his own preferences I feel obliged to offer him residency as he requires.
I have 2 older daughters (21 at university & 17.5 residing with mother).
My ex-wife has since harassed me, my wife and son repeatedly insisting he returns to her, which I am unable to influence. I have bought him a phone to encourage direct contact with her and invited her to discuss with me and son but she is resolutely angry and says this is not his choice or mine. There is no constructive dialogue and she has recently approached the CSA to enforce maintenance collection unilaterally unknown to me until they called. There is no history of maintenance difficulty however I did write to her to ask for a review now he was with me all the time. She has refused to offer him any clothes or property from her house.

I have not been able to engage her with a third party or with son to mediate a discussion on meeting son's preferences until her solicitor made a referral on her behalf.

I agreed to attend mediation to further discuss however she has now applied to court for an emergency directions hearing, claiming I have wrongfully reversed residency and he is at risk as he has failed to attend scouts on a few occasions. The first mediation appointment was due next week and has now been undermined by having to attend the directions hearing at the very same time!

My son is happy and healthy, enjoys the company of his father and stepmother a great deal. We are able and willing to care for him no problem.
What approach should I take to ensure his wishes and preferences are protected?

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Province/Country relating to question : UK