Logos, and Ethos
The author understands that politicians would want, and require statistics in his “report” and thus included a lot of numbers and facts on gun safety. Also, he maintains a professional feel while writing the letter, and thus increases his credibility as he shows that he is not joking about the matter at hand.
He has personal experience as a victim of the loopholes in the law, and therefore the author uses the fact to his advantage, by placing at the start and end of the letter that particular fact that his son was caught in crossfire and thus killed. This would make the politicians more sympathetic to his plight, and in the process get convinced of the idea that the law should be changed.