English native speaker?

My five year old son is an English native speaker, he speaks German but not as well as English. He had to take an English language test for entry to a bilingual school in Berlin, but they told my wife that they didn't consider him to have enough English. Reason? He hesitated when they asked him to do something or the other. They consider him to be a German speaker even though they did not test his German (and his scores for German from the Kita puts him at the lower bound for German competence).  Another reason they classified him as a German speaker is that I have a German passport. Obviously, if one of the parents has a German passport, German must be his/her native language. That's completely self-evident.

The arbitrariness of such decisions is sometimes too much to bear. This school doesn't even have an adequate process for detecting a bona fide native speaker of English correctly?

I should add that Atri went to another school the week before, and there he "passed" the test. I love randomness, but why does *everything* have a noise component? It seems like the entire course of his life could be set by the toss of a coin. No wonder the Hindus came up with the fatalistic "whatever happens is for the best." It's a coping strategy.

Update:

Next, my son went to the third school on the list, and this time he got 94%, which was regarded as performing "extremely well." I just can't understand how he can be considered a non-English speaker in one school and a clear native English speaker in two other schools. Doesn't this mean the selection process is flawed? A further issue is that Atri often uses German words for things that he encounters often in his German Kindergarten but not at home (e.g., Kerzen). Code-mixing is totally normal in bilingual or multilingual settings. So it's normal that a bilingual kid cannot think of the corresponding English word on demand. I wonder whether testers in Berlin schools understand this.

Incidentally, one cannot ever contest the result of a decision like this; if they decide your son is a native speaker of German in a particular school, he's going to be classified as such at that school, even if he is disadvantaged for the rest of his life (I recently learnt from a prof specializing in child education that it's critical that the kid learns reading in his primary language). No second opinions are possible. Isn't that amazing?

Basically, Germany stumbles every time it hits a multilingual setting, where everything is fluid and nothing is "purely" X or Y. Multilingualism lies outside the German experience. I hope this changes one day.