He could be teething and experiencing teething pains hence rejecting his usual foods. My girl went through that stage and rejected her favourite porridge, only taking bread and biscuits for some time. After that stage, she doesn't fancy porridge any more and turned to noodles, rice and pasta. If he's salivating more than usual or keep putting things into his mouth, high chance is he's teething.
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Mama to a cheeky monkey