wolfchan, he was probably trying to go as light as possible to reflect excess heat, and that means he was really bad off. He was probably trying to let his body shut down and die more quickly than starvation would kill him. Not basking would hasten the process.
It was a terrific thing you did taking him in and giving him a chance. Just seeing the color come back means he is turning around and wants to fight for his health now.
The Zombie Mama is here!