You mentioned nothing has changed, same people, same food etc.......
I reckon you have nothing to worry about.
Birds do go through stages, just like us humans, perhaps he wants quiet time.
When Mishka (my African Grey) began talking he never stopped for weeks on end.
There were days thereafter he hardly said a word. There were many times I thought he just did not want to talk, ever again. I continued talking to him, and never stopped. He began talking softly when I was not in the same room, the moment I entered he became very quiet.
It took him about 2 months before he wanted to talk again, and since then he has not given me a moment of silence................... music to my ears.
Keep things the way they are, talk, talk and keep talking, sure he will come around soon.
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