My son is 3 years old, and started to speak late. Now his vocabulary is vastly increasing, and I can ask him about colors of things or numbers or where is this and that, but I would like to pose him questions so that he can answer yes or no. What is the best approach for this?
My daughter refused to say yes or no for a while. She would answer with "uh huh" or "uh uh", which is hard for people to understand from a 2 year old. We sort of created the problem by accepting the answer. But people outside of the house had no idea if her answers meant yes or no.
So we decided to no longer entertain those responses. We didn't change anything about the way we asked questions. We just refused to accept those answers. If I asked her "Do you want to go to the park?" and she said "uh huh", well then I guess we didn't go to the park. Because I don't know what that means. I would say "I guess we aren't going to the park because I didn't hear a yes" (or something along those lines)
Admittedly, the negative answer "uh uh" was a bit harder to break. But by pretending that you don't understand the answer, whatever it is, the child must adjust. Humans have a knack of evolving to the situations around them. It is just like learning any language. If they can't get what they want with the communication told they use now, they will adjust to whatever communication works.
So I would suggest to stand firm, and get everyone on board with the plan. Do not even acknowledge head nods or grunts or anything until that kiddo starts saying yes or no.
Good luck to you.