Would you let your 5 years old son wear a dress?

If he's just at home, then yes, certainly. If he wanted to wear one at school, I'd subtly ask him why he felt the need to wear it and if it was a fundamental desire in him and made him happy, then I'd have a word with the school to allow him to wear one. I'd also explain to him the likely reaction he'd get from the other kids, and also from ignorant adults, and if he still wanted to go ahead, then I'd support him. It's basically a piece of cloth around his body. Up to the late 18th century, dresses were regularly put on boys. Styles of clothing are just a social convention. People hated the way hippies and punks dressed. We've all been socially engineered to conform.