When the son of an overly protective mother is told he has terminal cancer he decides to run away from home to experience life rather than spend his final days dying under the watch of his smothering mom.

^ I agree 100%. "To live" is such a vague, subjective term. There has to be a clear, concrete, visual goal that you can say "my protagonist is trying to do THIS ONE SPECIFIC THING, which will signify to the audience that he has lived his life."

NP, hope I helped

Title: Dying to Live

I suggest he ought to be running away not just to live more fully in general, but to fulfill a specific dream (objective goal). As to the nature of that dream -- well, there are so many choices. Take your pick.