Is it permissible for my eight-year-old son to lead me in prayer, and should I pray with him and then repeat the prayer?

The correct view is that it is permissible for a boy to lead adult men and women in prayer, without needing to repeat the prayer, provided he performs the prayer well.

Those who permit this cite the hadith of Amr ibn Salama, who led prayers during the time of the Messenger of Allah, peace and blessings be upon him, when he was a boy of seven years old. They also cite his, peace and blessings be upon him, saying: "The people are led in prayer by the one among them who is most knowledgeable of the Book of Allah," and that whoever's imamate is valid for supererogatory prayers, it is also valid for obligatory prayers.

As for the hadith of "the Pen has been lifted," what is meant by it is the lifting of religious obligation and incumbency, not the negation of the prayer's validity.