He is going in and out of falsetto, throughout the song. It gives the song a good dynamic, imo, but it is not an easy thing to get right, especially against the constant solid tone of the guitar, which doesn't seem to be following the same to-and-fro dynamic of lightening and consolidating.... I think he has to get the transition from "chest voice" to "mix" and "falsetto" a bit smoother. It sounds to me as if he sometimes hasn't decided which mode he will sing each note in, and therefore the tone isn't balanced, especially where the changes are all happening quickly.