I would give him some time.
DD started German kindergarten aged 3yo and at the time was hardly speaking German. I speak English, DH German and she understood German but spoke mainly in English.
She was a bit delayed in her speech, compared to other children her age.
Now, at almost 6, she speaks fluent German. More fluent than English as she is all day in kindergarten.
DS has just started kindergarten 6 months ago and speaks German most of the time, although he understands English.
He has the similar "problem" that your DS has, he does not know how to react when a child shouts at him. The kindergarten teacher asked me recently about his English, saying that they thought that he was thinking of a response in English, then not being able to translate it quick enough and getting frustrated.
I am 100% sure that this is not the case, no way he thinks in English, he hardly ever speaks English.
Tbh, unless the nursery has experience with bilingual children they sometimes tend to attribute any problem that the child has with the bilingualism.
There is a very good website about bilingualism, I will post a link when I remember the name of it.