It's a separate issue.
The rights given to transgender people largely stems from the (faulty IMO) view that transgender people can effectively become indistinguishable from members of the opposite sex.
See eg this from the Goodwin judgment (which led to the GRA 2004):
82. While it also remains the case that a transsexual cannot acquire all the biological characteristics of the assigned sex (Sheffield and Horsham, cited above, p. 2028, § 56), the Court notes that with increasingly sophisticated surgery and types of hormonal treatments, the principal unchanging biological aspect of gender identity is the chromosomal element. It is known however that chromosomal anomalies may arise naturally (for example, in cases of intersex conditions where the biological criteria at birth are not congruent) and in those cases, some persons have to be assigned to one sex or the other as seems most appropriate in the circumstances of the individual case. It is not apparent to the Court that the chromosomal element, amongst all the others, must inevitably take on decisive significance for the purposes of legal attribution of gender identity for transsexuals (see the dissenting opinion of Thorpe LJ in Bellinger v. Bellinger cited in paragraph 52 above; and the judgment of Chisholm J in the Australian case, Re Kevin, cited in paragraph 55 above).
83. The Court is not persuaded therefore that the state of medical science or scientific knowledge provides any determining argument as regards the legal recognition of transsexuals.
Or this from A v Chief Constable of West Yorkshire re: whether a transwoman police officer could search women
In my opinion, effect can be given to the clear thrust of Community law only by reading “the same sex” in section 54(9) of the 1984 Act, and “woman”, “man” and “men” in sections 1, 2, 6 and 7 of the 1975 Act, as referring to the acquired gender of a post-operative transsexual who is visually and for all practical purposes indistinguishable from non-transsexual members of that gender. No one of that gender searched by such a person could reasonably object to the search.
Whereas your DS will not be pretending to be a child or trying to become one. Your issues will be whether it is a reasonable adjustment for the event to allow access despite his age in light of his disability, which is a separate topic.