Besides death/dying, belly up/exposed is a sign of being too hot in my experience.
What are your temps? That pink belly concerns me if he's not in the blue.
If you have an under tank heater (UTH) that is not regulated, unplug immediately!
Do you have a temp gun to accurately measure temps?
IF heat is the issue, this is one of the few times a soak can help, IF the water is cool to cold. This will suck the excess heat out of him. Water is about the best temperature delivery medium.
Sure hope he's not too far gone.