What Does Joe Brody Say?

I always thought he was repeating what he said in English to the Japanese guard since he would know where his son would be.

I'm actually taking a Japanese class, so:

Musuko (the extra u is silent) - "Son" (though as it is not a respectful version, it usually referes to "My son")

Doko - "Where"

Da - an informal version of Des, which means "is"

So, effectively, he's saying "Where is my son?"