I would be somewhat surprised if he was actually stuck, frogs can compress their bodies a great deal and he doesn't looked compressed anywhere near enough to be stuck. I consider it much more likely he just doesn't want to come out for some reason, possibly because of stress or because things are to dry and he is trying to find a spot to conserve moisture. The fact that he is able to retreat deeper is also something that indicates to me that he may not actually be stuck. If upon careful examination I was firmly convinced he was stuck I would just carefully remove some of the surrounding wood. Before I tried that though I would just put the piece of wood in a covered five gallon bucket half full of water and leave it overnight to see if he would come out on his own. Bullfrogs, even young bullfrogs, are water loving. I wouldn't worry about the wet wood expanding either as it shouldn't expand enough to cause an issue.