Along his journey, he gathers strength and courage as he fights off wild animals, hides from the Forest tribes, and makes decisions on his own. Before, he was beholden to the rules and rites of his tribe; on the journey, he gains confidence, and feels okay in passing previously set boundaries. He takes things further than anyone else in his tribes has done before, and as a result, gains more knowledge than anyone else.
In the end, it opens his eyes to a great truth, that the gods were men, just like them. This gives him incredible knowledge and power; then, when he gets home, upon the advice of his father, he tempers that knowledge with wisdom, choosing to dispense his knowledge a bit at a time, in order for his people to use each bit wisely.
So, he left his tribe a boy who saught permission to do all that he did. He was naive and expectant, excited and had a very limited perspective. He came back full of confidence, having passed numerous tests of strength and courage, and armed with knowledge that would change him and his people forever.