Singing tuition is usually given on a one-to-one basis (and therefore tends to be more expensive than drama classes). The easiest way to find a singing tutor is to look through the ads under 'Music Teachers' in Yellow Pages.
If your son already has a good singing voice, then you should consider an audition for Manchester Boys' Choir. (My cousin used to sing with Southend Boys' Choir. He had a wonderful time including making records and singing solo in the Festival Hall). Manchester Boys' Choir can be contacted via Manchester Music Service, who may also be able to help you in your search for singing lessons. Contact details are here:
http://www.manchester.gov.uk/education/music/contact.htm
Hoping this helps,
Chris