If you want evidence that Reverend Hale no longer believes in the girls' testimony, all you have to do is look at the last lines he speaks in this act. He says "I denounce these proceedings, I quit this court," goes out and slams the door. Since the girls' testimony is the bulk of evidence that the court has been using, these words seem to me to prove that he no longer believes in that testimony.
You can also see evidence that Hale doesn't believe the girls a little earlier in the act. For example, he tells Danforth that he cannot see the "bird" that Abigail sees. He also points at Abigail and the girls and says to Danforth "you cannot believe them!"
All of this proves quite clearly that Hale does not believe the girls.