There's a list of agents here you might like to consider,
http://screenterrier.blogspot.com/p/agents.html
plus some general tips here:
http://straightfromschool.org/acting-gu ... explained/
but these are acting agents.
I'm not trying to be obstructive at all, but I wonder whether perhaps I confused you with my post on the money/fee front.
An agent is unlikely to be looking for (or even know about) classical music opportunities for child clients because there is no money in it
for the agent, so you would still need to do your own hunting for this kind of opportunity. Coco's suggestion of regularly searching the internet is a good idea. You can also keep a close eye on what is happening at any professional theatres near you just in case there are opportunities or any contacts you can make there. Music and Arts Festivals are another possible source of opportunities - there are quite often community/youth opportunities advertised. National Association of Youth Theatres is another potential source of information.
I'm not certain whether an acting agent would take on a good singer unless he/she also had good acting skills/experience - you'd obviously need acting skills for a West End musical unless it was something like Joseph and His Amazing Technicolor Dreamcoat (and in this case, the choirs tend to be from local stage schools like Stagecoach for the touring show I think). Of course your son may have great acting skills/experience - in which case an agent might well be interested!
If you are starting out it's important to recognise that, however lovely an agent is, he or she is in the business to make money. They're not there to facilitate children having a lovely time enjoying their hobby or improving their skills. They will either make the money because they charge you to be on their books (and therefore don't have as much incentive to try to find work for you in my opinion) or they will make money because you/your child earn money and they take commission. In either case, they probably won't be terribly interested in (or well-informed about) any kind of amateur opportunity.
Does your son have performance experience other than as a singer Talentless Mum (that feels all wrong typing that
)?
What kind of things do the other members of the choir get involved in that you think your son might also like? If you can give me a general idea of the sort of thing they do, I might be able to suggest where you could find out about similar opportunities.
BTW have just posted details of Gilbert and Sullivan opportunities in the summer holidays in Derbyshire on the auditions page - don't know whether this will be of interest or within reach!
I'd be really grateful if a kind member would pm me the name of a suitable agent...Coco? 
I have trawled the internet and I do not know where to start.
and he got put in with the 12 year olds because of his height (he was 
. He also struggled trying to make his "posh" singing voice sound cockney, but as he as got more moody this seems to be happening more naturally.
Anyway I will definitely look into all the suggestions and good luck to all your DC's, who seem to be doing very well. 
)