To block your ex-partner from taking your son to live abroad you must have 'parental responsibility'. You would have automatically have acquired parental responsibility if you'd been married to your partner. If your son had been born in recent years (since 01/12/03 in England & Wales) you could have also have acquired parental responsibility simply by jointly registering the birth with your former partner. However, since those provisions don't apply to you, you need to seek a court order to obtain parental responsibility.
Once you've been granted parental responsibility, the provisions of the Child Abduction Act 1984 will allow you to prevent your son's mother from taking him out of the country for a period exceeding one month:
http://www.ips.gov.uk/passport/apply-child-rul es-parental-responsibility-rules.asp
and
http://www.direct.gov.uk/en/Parents/ParentsRig hts/DG_4002954
Chris