Myview2
Myview historically the scouts was a single sex organisation. The guiding movement was started so that girls would also have an organisation.
Over time the number of scouts fell so they decided to go unisex. Guiding did not have the same problem so remained a single sex organisation.
Now the good news for your son is that until recently the guides have been single sex and are allowed to be as an exemption under the equality laws to prevent male sexed children to become members.
They have now become a single gender organisation. While there are legal protections/exemptions for excluding members of the opposite sex there is none for excluding different gender. (Gender is no longer a synonym for sex it is some sort of internal feeling)
Therefore if your son really wants to join the guiding movement he could claim they are discriminating against him as they are no longer applying the single sex exemptions and there are no single gender exemptions allowed.