I consulted expert before on

this issue.

If that's his only speech and language problem, then you don't need to bring him to a speech therapy. (Insurance won't pay if it is not serious).

The expert told me that when he mispronounce a word, you just casually say,"you mean Lai not nai, right?", you say the correct sentence for him. Play some games and practice that prounciation.

Don't be too harsh and give him pressure. Otherwise, he may feel inadquate and become reluctant to talk.

It's not a big deal, he will outgrow it. I kind of enjoyed my little one when he reply"呆“ after we ask him 乖不乖.