Frédéric Chopin:

In 1817 the Saxon Palace was requisitioned by Warsaw's Russian governor for military use, and the Warsaw Lyceum was reestablished in the Kazimierz Palace (today the rectorate of Warsaw University). Fryderyk and his family moved to a building, which still survives, adjacent to the Kazimierz Palace. During this period, Fryderyk was sometimes invited to the Belweder Palace as playmate to the son of the ruler of Russian Poland, Grand Duke Constantine; he played the piano for the Duke and composed a march for him. Julian Ursyn Niemcewicz, in his dramatic eclogue, "Nasze Przebiegi" ("Our Discourses", 1818), attested to "little Chopin's" popularity.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". As a child Chopin was invited to play with the son of whom?
Grand Duke Constantine