Hi, thank you for your question. Is your son in England or Wales?
Thanks for confirming. As he lives in England, the English courts will have jurisdiction however, given he is now 16 years old the courts will not deal with any application regarding him (under the Children Act 1989) as the cut-off point is 16. The courts will only consider dealing with children matters up to 18 if there are exceptional circumstances, such as if he has disabilities or special needs.
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The reference you are looking for is here:
Section 9 - Part (7)
Section 9 details the restrictions in respect of orders sought to spend time / live with a child. Part (7) confirms that this ends at 16.
The mother and her new husbands application to court would therefore not be entertained by the court if she tried to take such action.