Lawyer-01's answer is incomplete.
If you've not got 'parental responsibility' you can't stop your ex from taking your son out of the country irrespective of whether it's for a short holiday or for ever.
You will have parental responsibility if
(a) you were married to your son's mother at the time of his birth ; or
(b) your child was born was born (in England or Wales) on or after 01/12/03 and you jointly registered the birth with your partner ; or
(c) you've entered into a formal parental responsibility agreement with your son's mother ;
(d) you've been granted a parental responsibility order by a court.
If you've got parental responsibility, your ex can't take your son out of the country without your permission (even for a day) unless she has a residence order. If she has a residence order she can take him out of the country for up to one month at a time without needing your permission.
Relevant links:
http://www.direct.gov.uk/en/Parents/ParentsRig hts/DG_4002954
http://www.ips.gov.uk/passport/apply-child-rul es-parental-responsibility-rules.asp
Chris