No, ‘when’ can not modify a noun. Here ‘when’ is an adverb and acting as subordinating conjunction to introduce the sub-clause “he was a child”. The adverbial clause is modifying the adverb ‘something’ in the main clause. Look at a few similar examples:
She was looking something like her mother when she put on that new dress.
He took on something unusual about me when I was absent.
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