Multilingual Children

My son is 4, 5 in July. His father and I are both English and we live in Spain. We have spoken to him only in English since he was born here in Spain. He has attended nursery since he was 2 and started school in September. He has never spoken Spanish and even since starting school last year does not communicate at all verbally other than basic hello, yes, no etc. This causes problems at school as his teachers are not used to this and think that by now he should be speaking Spanish. It causes problems in the classroom and with his peers who don;t understand his English. He’s not shy just perhaps stubborn? I’m trying to ride it out and not worry but it does seem to be a problem that nobody else experiences here- help!