Lewis did not say he liked that order "much better".
He got the letter from that boy (who should never have written it in the first place now thanks to all the confusion and unimaginative re-ordering), and after reading it, he wrote him saying he's inclined to agree with him that he would prefer that order, if he were going to make some changes first. But we all know he didn't get to. And Lewis didn't have his publishers renumber them in the new dull magic-ruined order---they weren't renumbered until the '90s, and Lewis passed away in 1963.
So, I again stress from an earlier post of mine:
It's just so much more magical and enchanting if you read them in the order Lewis himself wrote them. Otherwise if you do it in the order which the publishers have rearranged them (which I personally detest), some of the magic and fantasy is ruined for you. It's like reading a textbook where everything is spelled out for you with no chance to wonder about things, question lovely little details, and those fantastic "Aha!" moments you get when you piece things together.
Besides, the way LWW is written it really is the only proper gateway to Narnia IMO.