We have discussed this in some thread. Seems to me he often uses a prerecorded and autotune'd lead vocal. Compare different live versions of the same songs and they are identical.
Here, in Take It Easy, the prerecorded track appears on the wrong beat when the final verse starts (I guess the drummer lost the click at some point) and Don must sing the rest of the song live. You can hear the difference. No Autotune and more pitch "problems":
https://www.youtube.com/watch?v=HegtSeolbcM
Don often misses the word "'cause" in the line "'Cause I'm already standing on the ground", but still we hear the word. Like here at 1:03 mark:
https://www.youtube.com/watch?v=QjUj8qF47xA
It looks impressive how Don always nails the high wails in the middle of Witchy Woman in the original key, but I don't think he sings them live. But like I've said before, personally I don't mind him using a prerecorded vocal track, because I don't think it's his singing people come to hear. It's the guitar playing.