What a wonderfully wise question from a six year old! Please accept my blessing that your son be a source of immeasurable nachat (pleasure) to you and to your entire family!
In truth, the answer to your son's question is not too easy to explain to a six year old since the reasons for the five “final letters” are esoteric and Kabbalisitic in nature. I will try to briefly explain the basic reasons but the more difficult, challenging (and fun!) part will be for you to try explaining it to your son!
Final Chaf: Instead of it being bent, the final chaf is long and straight. It is coming to teach us that someone who “bends” himself to do God's Will, and controls his desires and urges, will ultimately, with God's help, “stand up straight” to be morally and ethically upright.
Final Mem: This symbolizes the secrets of the Torah (it is closed on all sides). The letter mem, which is the first letter of Moses' name, is normally written with an opening, which denotes the “Revealed Torah”, but the two mems together represent the entirety of the Torah.
Final Nun: The letter nun is representative of the word "ne'eman", which means “faithful”. The shape of the final nun represents uninterrupted faithfulness to God.
Final Peh: The word "peh" in Hebrew means “mouth”. The two forms of the letter symbolize the power of speech. The regular letter peh represents a closed mouth, and teaches us a lesson as explained by our Sages: Silence is normally preferable to too much talking. The final peh represents the idea of speech and its importance, and the tremendous good that it brings into the world when used in the correct way.
Final Tzaddi or Tzaddik: The word "tzaddik" means righteous, and the regular letter tzaddi is representative of a righteous person in this world: One who is humble and unassuming. The final tzaddi represents the righteous person in the World-to-Come: One who is eternally upright and beloved by God.
