As you haven't been in contact with your son for a long time it may be difficult, as courts favour parents who have trid to contact their children or at least sent birthday cards, made phone calls etc.
Have you no idea where your ex and son are? the first port of call would be getting a solicitor to give you advice (get a free consutation) one who specialises in family law, of course.
They can advise as to whether it is worth you applying for a court order regarding contact. As you have been paying maintenance you have one foot in the door, anyway!
Try putting this Q in the law section as you may find experts there. Good luck :o)