The credibility shown by John Simmons was clearly displayed by the vastly wide knowledge on the content he was discussing about and the amount of information given though the letter. He also started off the letter by showing appreciation to the receiver which might boost his charisma.
For Pathos, the emotional appeal he gave to the audience by explaining that he lost a child in a gun accident and the fact about the number of firearms and what it can be used for. These facts is able to bring out fear in the audience knowing how dangerous the firearms are.
As for how John Simmons demonstrated Logos, he gave many examples and means of explanation for each fact that he gave, like showing what had happened to his son and that even his child had experienced, this is a vivid attention-capturing reasoning against firearms.