This is totally bonkers. I've seen about twenty of these 'at the seaside' dances courtesy of attending billions of dance shows and competitions with my now 16yo dd. In every single one of them, the girls have been in two pieces and the boys in trunks. They are on stage for about two minutes, bounce up and down, pretend to throw beach balls, and do that shimmy thing to pretend to go underwater. The more avant harder dance schools will have the teacher holding blue chiffon and wafting it about from the wings to simulate the sea,
They are all bog standard little kid dance routines, in dance inspired swimming costumes (yes, unfortunately if your ds continues in dance, he is going to have to get used to wearing a leotard - will you ban that as well?)
Ds danced for years. He was also the only boy on his dance school. On one occasion at four he had to wear a silver sequinned vest top and mermaid skirt. It was freaking hilarious.
I promise you, swimming trunks is entirely normal in this scenario. Big ol' skate shorts would not be appropriate - not unless the teacher has envisaged a comedy character for a solo part at some point during th routine - the idea at 4 is that he will blend/ match the girls in their skimpy cossies.
At our dance school they get given their costumes. There is no 'oh just send him in something else'. There is a vision, darling. You can't just decide to do something different.
Still lol at the pearl clutching. At 6 ds was a farmer with a herd of dancing sheep. At 7 he was Austin Powers with a troupe of girls in mini dresses and feather boas. Last year at 14 he was in a tux with a tap dancing group of brides ('my future husband')
If he wants to dance, get him in those trunks and stop being a Nellie. They all have to go through the 'at the seaside' year.
it's a rite of passage.