This thread is pretty circular. I think one issue is that some people can't imagine a little boy choosing a dress and assume it must be the work of an attention-seeking mother. I can see why that would get a more dismissive response.
Also, some people are clearly much more prescriptive about school run attire and perhaps that's dictated by the norms of the particular school. At ours, little boys wear fancy dress and no one bats an eyelid. They tend to be all in one superhero suits so fairly practical. Consequently, I'd let a child of either gender wear weather-appropriate fancy dress on the school run. I wouldn't mind about it getting dirty or ice cream spills because as far as I'm concerned, play clothes aren't for keeping 'nice', they're for playing in so do have a shelf life. Most are machine washable though and manage just fine. I think play clothes don't need to be limited to home or parties because up to school age, kids are playing all the time so play clothes are fine all the time. I wouldn't allow a long dress on a climbing frame or similar for safety reasons. That might be why I see far more boys in fancy dress than girls because with the exception of one little girl who I've seen in a sparkly Spider-Man (Spider-Woman?) mask and a pirate outfit, all the little girls round here dress up in Disney princess dresses when they dress up so maybe they get worn less for these practical reasons. If you're not going climbing on the school run through, it's fine.
I haven't seen little boys out and about in dresses here but I have seen them dive on the dresses and sparkly shoes in the pre-school dressing up box and I've never seen an adult or child comment negatively on this.
I'm not especially fussed about small children dressing smartly for weddings. The last wedding my son went to, he was 2.5 and I put him in a nice shirt and trousers. At the swelteringly hot reception, he frantically started pulling them off and I put him in a shortie cotton pyjama set for his comfort. The only comments we received were adults wistfully remarking that they wished they could do the same!
I can see that if you are likely to meet ridicule that you would dissuade your son from wearing a dress. But I think if you personally find a small boy on a dress ridiculous or distasteful, this springs from nothing but prejudice, however you try to dress it up or justify it as coming from concern or protectiveness. Children tend to be much more tolerant and accepting than adults - we are all responsible for not teaching them prejudice and intolerance.