Can Red Eared Sliders Breathe Underwater?

I have a Red Eared Slider which i got from a friend. He's about 4 inches long. He always stays underwater. Can he sleep underwater? If i put a Styrofoam piece in his tank, will he climb on top and sleep there, or will he always sleep under the water? Because I’m afraid that if he can't sleep underwater and cant climb on top, he will die of exhaustion. Or what else can i setup in the tank so that he will be able to climb up and come on top to the landing area? Would a pile of rocks or such be advisable? And how often is it recommended that i feed him? And how can i estimate his age? Any answers to these questions will be very much appreciated. Thanx...