I can't help you with any specific info on coaches or South Eastern rinks in general as I've sadly never been to any. What I did do was try to assess which rinks male skaters of your son's age came from by analysing the recent male entrants to UK competitions, particularly ones held in the South East*. I wrote a lot of stuff originally, but the take home is basically this: whatever rink you end up at then your son will probably meet two or three other guys his age. There aren't a lot of guys in this sport in general (my own rink, Oxford, only has three or four at your sons age, compared to dozens of girls), but there are generally a few at each rink.
* Analysis based on competition entries is a bit flawed because not everyone competes, but it's the best I had to go on to indicate where has male skaters.
Based almost entirely on hearsay from speaking to the younger skaters at my rink, I know that there's a popular male coach in Chelmsford who coaches several achieving, young male skaters - I've no idea what his name is though.