You need to apply for a specific issue order from the family court, in this case.
If there is no reason why this would be detrimental to your son then the court should grant the holiday travel.
Unless this is very urgent (i.e. you have tickets booked for imminent travel), you will need to see a mediator first though.
The process is started by booking a mediation assessment appointment - see here for a link to find your local mediator:
http://www.nfm.org.uk/. Assessments are usually around £30-£40, and the mediator will advise as to how many sessions might be required.
Following that, if the mediation does not succeed (which it may well actually do, as the mediator should be frank about the slim chance of your ex being able to successfully oppose the application), you can apply to the family court for the specific issue order, setting out the holiday proposal.
This is on form c100:
You send 3 copies of this form to the local family court centre. There will be a court hearing after you send the form in and a family court adviser will be there in attendance too (from CAFCASS).
The application costs £215.00, and you can represent yourself so you don't need to engage a lawyer.
Can I clarify anything for you?