His face looks like his dog just died. It's either something made him emotional, or Kishi made a mistake. But I don't think Kishi makes these kind of mistakes.
I also don't think that he thinks he's weaker than Naruto, but it's something more complex, something related to Naruto's relationship with his dad.
The Alliance just saw a small portion of Naruto's power and they can't believe it,Kiba is upset at Naruto's abilities all are in amazement at Naruto's power and smarts. All but one person who always believe in Naruto from the very started, Hinata who always had faith in Naruto.