Why does the clothing in The Blue Boy resemble the portraits of Anthony van Dyck?

The clothing in The Blue Boy resembles the portraits of Anthony van Dyck because Thomas Gainsborough was deeply influenced by the Flemish master’s work.12

Gainsborough held Van Dyck in high regard, considering him a hero and a primary influence on his own portraiture style. By painting the subject of The Blue Boy in "Van Dyck dress," Gainsborough was intentionally paying homage to his predecessor while also following a popular 18th-century fashion in painting. This stylistic choice allowed Gainsborough to adopt a more formal and elegant approach to portraiture, mirroring the dignified simplicity and refined aesthetic often found in Van Dyck’s own compositions.1