It is quite clearly a growing up thing, just looking at the last lines:

That day, the last of my youth, on the last of our mountains.

But more it is about David letting go of Bob(narrator). In the beginning there are many David showed me this and David taught me that, showing us that David is in fact the elder here, as said possibly the father. Also the ending is highly hinted in a smaller metaphor with the robin:

And drew me up after. We set a new time for that climb.; That day returning we found a robin gyrating; In grbum, wing-broken. I caught i to tame but David; Took and killed it, and said, “Could you teach it to fly?”

Clearly robin represents Bob here, not able to fly/become an adult/be free, and and Bob would be the robins tamer/teacher, and David is the mountain. There is no way the Bob could teach anything more to the robin to make it free, as there is nothing more David can teach to Bob about adulthood. The suspense here comes from the fact that the robin is killed, not the teacher, thus posing the question if bob, our narrator is killed in the end.

When David gets injured, it is clear to him, instantaneously that he will die, and first thing he wants to be pushed over. He has to let go of Bob, and he knows it. And as it continues:

Straight eyes and brokenly murmured, “over… over.”; And I, feeling beneath him a cruel fang; Of the ledge thrust in his back, but not understanding,; Mumbled stupidly, “Best not to move,” and spoke;

of his pain. But he said “I can’t move … If only I felt; Some pain.” Then my shame stung the tears to my eyes; As I crouched, and I cursed myself, but he cried; Louder, “No, Bobbie! Don’t ever blame yourself.”

First off I´d note that this is the only point in the poem where between the paragraph thingies there are two lines in stead of one. I doubt it is an accident. This is the vital turning point in the poem, but I won´t dig into this too deep.

Bob does not understand the situation, he doesn´t understand that David has to let go, thus he tries to save him. Also David said I can’t move … If only I felt Some pain., in which the pain pretty much represent the feelings of him letting go of Bob, and that he feels none means he has no regrets and is in fact happy that he can now let go, and Bob can live his own life. also in the story’s context it means that he is happy it is him dying, not Bob.

The same goes to the last line. Bob is never to blame himself for what it takes to let go, to grow out from under someones wing. Also to top this of David is unable to push himself over the edge, Bob has to do it. Bob is the one who has to let go in the end, not David. It is really quite touching understanding that the mentor has to let go first, has to understand that there is nothing more to teach, but in the end the student, bob here, has to truly let go.

Anyway that my two cents, I haven´t done this in a while but it was refreshing. And from this you got at least the robin thing, and the letting go bit in the end as far as metaphors go.