The writer starts of by praising the congressman, to allow the congressman to have a good impression about the writer. The praise was also used to draw the attention of the congressman so that he would read the letter and not ignore it.
By stating in the letter that the writer was a parent who had lost a child to a gun incident, makes him more credible. As he had experienced the consequence of the absence of a strict gun control law, the congressman would more likely take him more seriously and consider his point of view.
He also stated the likely argument of the opponents of gun control, and went on to state why their argument was not credible. This shows that he is fair and not one-sided, and also has researched well into this topic before he wrote the letter.
The writer used a lot of figures to support his point of view, and not just bluntly stating his opinions. He used credible sources like the FBI and evidence from the Kellermann study, instead of websites from the internet, to show that his opinion was backed up.
He also used the phrase "Did you know" to make the congressman more aware of the fact, as he most likely did not know about it in the first place.
Pathos (appealing to the emotions/values):
The writer also appealed to the emotions and values of the congressman by telling him of the consequences that would come if laws on the ownership of guns were not tightened. He also asked a few thinking questions, to further persuade the congressman to vote for stricter laws.