A childs passport needs to be renewed every 5 years until the age of 16, then they're classed as an adult, the passport will then be valid for 10 years.
He may need to apply for a new passport by the sound of it - is it out of date?
You need to allow at least 6 weeks for a passport application to be dealt with and then for a new one to be sent back.
You could speed it up a little if you go for the passport check & send service that is carried out by some of the post office branches. They will send it off by fast track & you'll get your passport back within 2 weeks.
Because of the change in appearance of the child, section 10 of the application form will need to be filled in by someone who has known him for at least 2 years (teacher, doctor etc..) and one of the photo's will have to be certified & signed by that person too.
The background colour has to be either light grey or cream in colour - they'll reject it if it is white (changed rules in October 2008)
here are some useful sites:
http://www.ips.gov.uk/passport/apply-child-rul es.asp
http://www.ips.gov.uk/passport/apply-post-offi ce.asp
http://www.direct.gov.uk/en/TravelAndTransport /TravellingAbroad/PassportsAndVisas/DG_1001498 4
hope this helps