Other than the commas, is the sentence unnatural because of the part "much praised"?Some commas would clarify it: the boy, having won a prize, was much praised. That would make it grammatical, strictly speaking, but it would nevertheless be unnatural.
Such constructs are easily formed if one translates word by word from Latin to English, and my high school teachers never tired of telling us not to do that because it produces unnatural English sentences.
If you have a question about the English language and would like to ask one of our many English teachers and language experts, please click the button below to let us know: