«Help, my son has completely shut up!»

One question - three opinions
My son, 11, is in year 5. He has been a good pupil so far. Suddenly he has completely shut down and even refuses to do homework. I don't know the reason for this. His average grade is below a 4. I've cancelled his media consumption, but that hasn't helped. How can I get him to enjoy school and learning again?
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