Hai aditi. It's really very important to teach these things. The best way is during massage and bath. While massaging daily...keep telling. .pointing his parts that.. these areas should not be touched by anyone or shown to anyone. should not talk to anyone. Only mama will see, touch and take care during bath. Only mama will say who you can trust. Keep saying this...  he will know . Don't worry. Kids pick and remember things fastly .