I agree that his perception of time, being held alone in a cave, could very well be messed up. He was "born wrapped on darkness", maybe that's him sleeping/dead til the "right time".
But I don't believe that he was awake for only a few months. He got hurt climbing the rocks and recovered from it... Also, his hair at some point was very long, would his hair be growing when he was dead/dormant?
Also, Rachel taught him to talk, write and read, that's not a fast process and he was smaller on the flashback than on chapter 1