of who?

We are teaching Lancerlad to address people when talking to others.

In school, sometimes Lancerlad just says "Good morning" to the teachers. Only when we asked him "Good morning who?" then he would address the teacher's name.

Same thing when talking to our neighbours,
Neighbour: Hello, boy boy
Lancerlad: Hello
Lancerlord: Hello who?
Lancerlad: Hello, uncle

So just before we were going to sleep, I said "good night" to him.
Lancerlord: Good night
Lancerlad: Good night who?
Lancerlad: Good night, Lancerlad.