Yes I love that cover too! I wouldn't mind if he would sing lower from now on, like the pre-choruses of Last Party, it's delicious.
I agree with what others have said, taking singing lessons myself, he should keep training his falsetto to keep it strong. It's like sporting on a high level, keep that muscle trained and under constant supervision.
But perhaps he's just sick and it hit his vocal chords. Whatever it is, it's good that he's resting now and I hope he will get well soon and be in better shape than ever.