Kane was born in Chernarus before the civil war. His family lived on a farm where Kane had worked for most of his childhood. His family didn't put much value on his education as his father was physically limited, he wasn't too smart, to begin with, and he was much more valuable to the family working on the farm rather than in school.
When the civil war broke out Kane's family was scared that because of his physical prowess and lack of intelligence he would be easy pickings for the rebel forces to recruit, where they knew he couldn't last long. Because of his lower intelligence, lack of schooling and limited contact with the outside world, Kane was easily influenced, extremely gullible, and would after taking peoples words quite literal which would usually result in less than ideal outcomes.
In fear of losing their son, not because of their love for him, but rather their dependency of him for their farm, the family sent him to live in the U.S. with relatives until the conclusion of the war. The U.S. based family quickly realized Kane's shortcomings and how the family had been treating him, and even after the conclusion of the war, refused to send him back. Over the next 7 years, the family got him help, gave him an education, socialized him the best they could and helped him to become more independent.
As things became more clear to Kane and the family explained to him his upbringing, Kane eventually decided that he wanted to meet his family once more. See, Kane had a hard time understanding why the way he was raised was wrong. He has a difficult time understanding that people can be evil and assumes that any action is taken, or word is spoken is with the best intentions or the truth. And so Kane flew back to Chernarus in early April of 2017 to find his family who no one had heard from or seen for many years. What Kane didn't know is that his father had died during a particularly brutal winter having had worked himself to the bone since he no longer had Kane there to do the physical work. His mother died shortly after as she didn't know how to survive on her own and stayed in that farmhouse, depressed, alone, and eventually succumbed to dehydration and shock.
Kane flew in, happy as can be, looking forward to finding his family. He wandered the countryside for weeks until the wars started. The loud booms of the bombs were new to him and scared him. This situation was confusing to him, so he ran and hid among one of the many rural farms that were abandoned early before the outbreak. Kane spent the next few months sneaking around Chernarus, finding food where he could, and avoiding human contact as much as possible. He had seen things, violent things, cannibalism, murder, that he couldn't even comprehend. No matter how much his family in the U.S. had tried to help him and prepare him for life, who would've thought that these were things that he would've needed to know?
And so Kane survives in Chernarus, looking for food, keeping human contact to a minimum, and looking for his family. Though now Chernarus seems to be shrinking, there seem to be far fewer people, making Kane more comfortable to leave the shadows in search of the ever-dwindling food supply.