I agree about the patriarchy - rule of the father. Boys and young men may be worth more than girls and women, but they are worth much less than men.
Patriarchal societies treat young men as disposable, butchering many of them in wars. Patriarchal societies often treat widowed or abandoned mothers so badly that it creates huge numbers of homeless boys, as the mothers cannot afford to keep them and they cannot be married off.
To get back to the clothes thing. White frilly smock type dresses used to be considered unisex on toddlers. This is rather like now when many people would put a teddy motif on a young child of either sex and possibly an adult woman would wear a teddy print (they were popular on adult women's jumpers in the eighties), but an adult man would not wear something so heavily associated with infants or toddlers. So clothing changes over time. The meaning of loose white smocked dress was toddler then, not woman. Women's dresses were different in style (heavy corseting).
I don't think it is simple as just dressing boys in 'boys' clothes, or dressing boys in boys and girls clothes to avoid sexism, OP.
I would avoid the following for a boy - clothes that have violent connotations - camouflage, army style jackets and colours,
Clothes that are all about man made, technological things - car, robot, trucks, tool motifs.
Clothes with large motifs of animals that pose a threat to humans - close up of shark's jaw, large poisonous spiders, big snakes.
I see that kind of thing dominating boys' clothing displays in shops, and yet parents claim it is difficult to get appropriate clothing for girls, as if the boys' clothes are neutral and not an issue.
I tried to dress my son in clothes that had natural motifs - woodland, sea, companion species, harmless species so that his associations were some kind of harmony with nature. Alternatively whimsical and imaginative - wizards, mythical creatures. I didn't want to dress either my son or daughter as a miniature soldier or heavily associate them in childhood with the mechanical or the dangerous.
The taboo against boys in dresses is very strong, but reinforcing harmony with nature and imaginative ideas rather than violence through clothing is one way of increasing femininity and reducing masculinity.
Sorry, bit longer than I intended.