Teach him who he is. He is a descendant of Avrohom Avinu. Avrohom stood against the world & chose Hashem. Then Hashem chose Avrohom & his children forever, & he is one of them. Teach him the history of the Avos & their dedication to Hashem. Going down to Mitzrayim as a family & coming out as a nation, the nation that received the Torah, the nation that he belongs to. Get him to feel pride in that heritage so he will want to commit himself to the ranks of the Jewish people.
Then give him an easy mitzva to do. Give him a talis katan. Let him walk around all day (even with the tzitzis tucked in), knowing that Hashem surrounds him wherever he goes. Tell him Hashem is thinking about him 24/7. Tell him to keep Hashem on his mind from time to time during the day. He will feel pride & ask you for another mitzva and it’s explanation. Pesach is coming. Invite him to your seder. Learn a little about it in advance so he’ll know what to expect. Anticipate the best seder YOU have ever had.