Hi, thanks for your question. Just a bit more information required to fully assist you:
-Are you both in England or Wales?
-Are there any court orders in place regarding him, if so what are the terms?
Thanks for confirming. As there are no court orders there is no obligation to return him due to the serious concerns raised. However, when she is out of rehab she is likely to want to have him back in her care and in the circumstances you should consider pursuing an urgent application to court for a child arrangement order (for residence) using form C100 and a £215 court fee. Given the concerns and urgency you would not need to engage in mediation.
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