Migs - he won't be 'hanging around backstage' in just the trunks though - that would be plain daft. He and the girls will be able to have clothes on over their bathing costumes - something easily pulled off at the last minute, like a tracksuit.
Professional ballet dancers dance in very, very tight tights - but I don't think that takes away from their dignity or makes them targets for perves.
I honestly don't see why people are getting in such a fankle about a room full of people staring at the OP's son. It's a performance - the whole point is that people will be looking at the performers.
If the OP doesn't want her child to dance on stage in just speedos, she should first have a discussion with the teacher, and ask if there is a compromise - the sort of trunks that cover more, that several of us have linked to, or board shorts - and if she cannot get a compromise that she is happy with, she should pull her son from the show.
But that would be a real shame, because he will have worked hard for this performance, and won't get to show off what he has learned.