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There is no action that your son can take until the baby is born
Once the child is born the court says that a child is entitled to a relationship with both parents and the courts will enforce this if necessary
As a starting point your son could try and discuss matters now using Family mediation
If that fails he can then make the applictaion for a Child Arrangement Order as soon as the baby is born.
It is important that he is realistic in his expectations
In the early months contact should be short - but often, maybe an hour twice a week and then build up week by week.
He may find this website useful
Please ask if you need further details