@SirChenjins I missed the fact that you have a son rather than a daughter, which does change the balance of how this affects him personally, in my opinion.
Is it a sport where boys have a physical advantage over girls, or is it something like bouldering where it really doesn't make any difference and mixed sex participation is common?
If there is no real need for it to be single sex, then if you could be arsed to write to the school I'd say something like, "I was quite surprised to see this new sports group labelled in such a way. Whilst I applaud efforts to help the LGBTQ+ community feel safe and welcome at school, I wonder whether labelling a specific mixed sex sports group as being for LGBTQ+ children and their allies might give the impression that other clubs are not inclusive of this group, which would be a shame. It might also create a barrier to joining for any student who is privately questioning their gender identity but does not want any other students to know yet, which I believe would be the opposite effect to what was intended. Would it not be better to simply label this a mixed sex sports group, which is what it is, and aim to foster an environment where all students, regardless of their sexuality, sex, gender, race, beliefs, disabilities or lack thereof feel safe, included and welcome everywhere in school, and that all the other students are their allies in relation to whatever struggles they may be facing? For other sports where students typically play in single sex teams for reasons of safety and fairness, it would be really positive to have an additional mixed sex team for those students who want one, again without any specific reference to LGBTQ+ students or their allies. That way a child who does not feel comfortable participating in sport with members of their own birth sex can choose to participate in a mixed sex group without needing to label or out themselves, but equally, children who are transgender or non binary but wish to compete with members of their birth sex should be free to do so in the knowledge that these spaces are also inclusive and welcoming, regardless of their gender identity."