@christiangirl2017 Thank you, that's a great advice! Yeah, I am doing some stupid and funny stuff, such as introducing some words which sound funny for him, or playing games, just incorporating Russian words. But sometimes I forget to do this..
I have done a mistake. And didn't teach my son my native language - Russian. He is now almost 4, we live in Japan and he can speak English and Japanese. But I want to teach him my language too. What activities did you do if you were in the same situation?