what does fyodor call nikolai

According to the search results, Nikolai refers to Fyodor with the friendly nicknames "Dos-kun" in the Japanese recording and "Fedya" in the Russian recording. In addition, Nikolai also calls Fyodor "my dear friend" and gives him the nickname "Dos-kun". It seems that Fyodor doesnt have a specific nickname for Nikolai mentioned in the search results.