I don't think there's a single word for this in English, except perhaps for "opine", which is quite a different register to "estimer". I would suggest something like "he was of the opinion that", "he suggested that" or even "he claimed that" - something of that ilk, which implies he said what he thought, but it was only what he thought and not an established fact. I'm not a native French speaker though - hopefully you'll get some more suggestions.