Hi, I don't own a bullfrog so health wise I couldn't help you but when you say he stares out of the terrarium do you have any background or anything on the tank? Sometimes it helps if you cover 3 sides so he cant see so much outside of the enclosure. You could even just put some paper or something on the back and sides till you get something better. And I agree with Yuromastic that he's probably just adjusting, give him a couple days and imagine he will come around.