comprehension before production

We recently made friends with a family that doesn't speak much English. The 5 year old and my 3 year old son are having a lot of fun playing. He speaks about as much English as my son does Spanish, but because each of them understands more of the other language than they are able to produce, the two languages have not seemed to interfere with their ability to play together.

Yesterday we were out and my son was introducing his friend to another bilingual adult we know. He said, "This is my friend, ____" My friend turned to the other child and said, "Y cómo se llama tu amigo? My son answered, "My name is _____" We were all shocked. None of us expected him to understand the question - he is not being raised bilingually.