I don't think it's related to gender - maybe fewer boys learn an instrument?
Certainly the male young musicians I've seen have been brilliant.
Out of the three talent shows I have recently attended all have been girl heavy. Same went for those I attended at the International School my boys attended. I don't think it has anything to do with talent or numbers playing instruments. I do think there is an element of peer pressure.
Not to say some girls aren't shy and some boys extrovert. In some areas boys seem to dominate, sports for example, whereas girls perhaps less so. In general when it comes to music/talent shows I find boys in short supply.
Re this case if DS accepts he has to do it and has been doing it successfully then I would encourage him to keep on going and give massive reward for it. What doesn't kill you makes you stronger or some such phrase.