Hi Squid,
The trials Club that use Bracken Rocks is Mansfield Main MCC. The contact there is Karen Crabtree, its her Dad Terry Crabtree who would remember me from the ACU. This is an ACU club and they will take on new riders from 6 years old,.
There is the AMCA who also have a trials section. But I am pretty sure that the kids have to be 9 to be a member with them.
Take a look at both and see which one your lad wants to aim for.
Failing that if he still wants motocross then he is more than welcome to come and watch my club race at mx meetings when they start at the end of Feb.
My club has a new track over the top of Lichfield way, so its quite central for viewing.
Hope this helps
Morticia