what is a head voice?
I think there's a bit of argument about it, but I've always understood Head Voice to be the highest register, NOT in falsetto. Falsetto's above it, like Brian's singing in the bridge. An example of Brian singing in head voice would be his lead vocal on "Wouldn't it be Nice". He was tricky though, he would often go back and forth.
Some people consider Falsetto as just part of Head Voice, though. The way you can tell the difference is to create Falsetto, the body makes some of the vocal chords in the throat stretched out, and the wind blowing over the thin stretched out 'flaps' makes a high, thin sound. Head voice is 'full throat' and much more powerful.
Again though Brian confuses that because he's often so good at falsetto that it sounds full.
In short Brian Wilson is a fantastic singer
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Edit: I just went and read up on it a little bit... and basically there is a good argument on both sides. He sounds as if he's singing the bridge in falsetto, but head voice actually can go higher than falsetto, which is pretty shocking to me.
Try this exercise: Sing a note in falsetto, as softly as you can (low volume)... now, holding the same falsetto note, try to make it as loud as you can. Your voice will switch out of falsetto into head voice at a note you never thought you could hit in head voice...