The Transall Saga — Gary Paulsen

Mark's solo camping trip in the desert turns into a terrifying and thrilling odyssey when a mysterious beam of light transports him to another time on what appears to be another planet. As Mark searches for a pathway back to his own time on Earth, he must make a new life in a new world. His encounters with primitive tribes bring the joy of human bonds, but violence and war as well -- and, finally, a contest in which he discovers his own startling powers.