In The Other Wes Moore, the author says to the other Wes, "I guess it's hard sometimes to distinguish between second chances and last chance." What do you think he means?

He means that when you get a second chance in life, you don't know if you'll ever get another one. For all you know, it could be your last chance. It all depends on whether or not you take the chances you've been given. The author's last chance was when he tried to escape from military school, which at that time he really hated. Fortunately, he got his head down, worked hard, and became a highly disciplined young man.
When the other Wes Moore was arrested for the murder of Sergeant Bruce Prothero, that was his last chance. The difference in his case, however, is that he'd run out of road; he had no more options left in life. Unlike the author, there was nothing that this Wes could do to turn his life around.