In fact the boy is Umewakamaru: Matsuwaka's younger brother, who has come all the way to Togoku with his mother counting on Sota for help but was separated from his mother and yakko (varlet) Gunsuke by pursuers.
この少年こそ松若の弟梅若丸で、母とともに惣太を頼って東国まで流れてきたのだが、追手によって母と奴軍助と離れていたのだ。