Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first
Why did your son not tell his ex about the trip?
Who has she left the child with in the past?
As you realise his ex is being unreasonable and has clearly acted in temper.
Your son needs to arrange to see a family Mediator for a Mediation Information and Assessment meeting (MIAM)
If it is possible to discuss matters with his ex then that is good - but if not he will have to apply to the court for a Child Arrangement Order setting out the time that the child spends with him
This can then be enforced if necessary
Hopefully it will not come to that - given that things have worked well until now it may be possible for them to agree a Parenting Plan that avoids a court hearing.
he may find this site interesting
I hope that this is of assistance - please ask if you need further details