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If she is under 18, they can prevent her from seeing your son, and control her in many other ways. However, they cannot keep her from getting proper medical care, or it should be reported to Social Services. Whether it is definitely child abuse is up to the authorities.
Hopefully, these young people will soon be able to make their plans without any further interference. These people will be so sorry if they are ruining their chances with their future grandchild.
Your son cannot sign a Declaration of Paternity until after the child is born. Forms will be available at the hospital. Here are some FAQ's which provide a lot of information: