I think he had as much impact as anyone in his position could possibly have, if not way above average for a whistleblower.
The problem is that the public quickly turned on him, in no small part due to the PR strategy of those who DO have the power to change this. And most of the public shrugged and said what are we gonna do?
His contribution was proof that suspected domestic spying under the guise of homeland security was being widely misused. Which is a violation of YOUR constitutional rights. Of course anyone paying attention suspected it before, but he brought receipts. Many people think he should be in jail or worse because of this.
Unless he's lying, and I don't think he has been shown to lie, he did attempt to use proper channels. The media has historically been called the 4th branch of government for a reason - and ultimately that's where he turned.
The man effectively lost much of his freedom and the people he did it for were unable to take it any further.