“he let none of his words.” That is, God let none of Samuel’s words “fall to the ground,” that is, fail to come to pass. God called Samuel as a prophet, and a hallmark of a genuine prophet was that what he said comes to pass. So God supported Samuel by backing up what he said and making sure his words came to pass. This interpretation is also supported by 1 Sam. 3:20, because all of Israel knew that Samuel was a prophet. How? Because his words came to pass.
“fall to the ground.” An idiom for “go unfulfilled.” 1 Samuel 9:6 makes the point that Samuel’s words come to pass.