I have to confess that while I have much admiration for CS Lewis, especially his theological science fiction (The Space Trilogy: Out of the Silent Planet, Perelandra and That Hideous Strength, I found his most popular theological work, Mere Christianity, to be much less spiritually edifying and nurturing than those works by Metropolitan Kallistos Ware, who was a student at Oxford in the 1950s who occasionally passed CS Lewis in the hallways. This being Oxford in the 1950s they never conversed; Metropolitan Ware has expressed appreciation for the writings of CS Lewis on Christianity but has no idea how he felt about the Orthodox Church. CS Lewis and his brother rose early to drive to a small parish church for a said Communion service every Sunday, and would leave immediately after partaking, not remaining for the Gloria or the Benedictio; based on this, it is unknown what he would think of the Orthodox Church where all services are sung.
Regarding Mere Christianity specifically, my main problem is the "Mere" bit; its just a tad too generic for me. However I dound its discussion of the flaws of dualism the most concise and compelling argument against that error I have seen. However much of the writings of Lewis do express a more doctrinaire Anglo Catholicism, or at the very least, High Church Anglicanism, and here we see Lewis at his theological best, and indeed it is largely on the basis of writers like CS Lewis and Percy Dearmer that I feel compelled to regard that portion of Anglo Catholicism that was not Anglo Papalist in the mode of Dom Gregory Dox to have been fully Orthodox, and I strongly suspect Continuing Anglican provinces of an Anglo Catholic disposition like the Anglican Province of Christ the King of Orthodoxy. As for CS Lewis, I hope that hhe might at some point be found to be glorified, and perhaps become a modern day St. Isaac the Syrian, in that while not a member of the Eastern or Oriental Orthodox communion, his weitings have been greatly edifying for the faithful (I myself actually prefer Patriatic writings, like Against Heresies, On the Incarnation, the Philokalia, and so on, and for his part, so did CS Lewis, who regarding theological books stated those older works that passed the test of time ought to be favored over newer works).