There isn't any mon or design that announce's the boy's identity, but the color scheme is clearly the clan colors of the Kaiu Clan - one of the city's Great Clans. The Kaiu boast that "We Are The Wall", and they make good on that claim as some of the city's foremost builders, engineers, and stonemasons. They maintain the city walls and are experts in fortifications.
Spoiler: Knowledge (Nobility) or Knowledge (Local) DC 20
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But they're not only builders of fortifications - as the foremost builders in the city, they've constructed significant portions of the city over the years and are major landlords. Much of their political power comes from owning marketplaces and being able to decide which businesses to rent space to.