Oh, gosh. There are so many quotations showing Cloyd's loneliness. How about this one, from very early in the book (page 2 in my edition)?
"When Cloyd was little, he used to talk to his sister about how badly he wanted to find his father, but she didn't seem to know him at all, so he had kept his dream inside. In the year since he'd been sent away, the more lonely he became, the stronger his desire grew to find his father."
"In the year since he'd been sent away, the more lonely he became, the stronger his desire grew to find his father."
This quote accurately demonstrates how lonely he was as his father was not by his side, so this portal of communication and love was cut away from his world, making him feel depressed and lonely, so this fuel his courage and desire to find his biological father as his father was the pillar of his success and his life.