So you agree that he loses the right to his body in exchange for the rights of this new head?

Okay. Interesting.

If he gets to it before the head has any conscious thought, then yes.

The new head isn't really "his" body. It has it's own consciousness and so on. It's not the same as cutting off his leg for example. He would murder another person.

I meant if he cut around his own shoulder without ever touching the new head. 

Oh...okay, now I get it. That's a difficult question. His shoulder is still his shoulder, so he should be allowed to do whatever he wants with it. But on the other hand, he knows that if he would cut off his entire shoulder before the head could detach itself, he would kill the head and it should be considered murder. Like I said, he knew this could happen so I think the rights to his own body are limited. He simply shouldn't be allowed to do something to his own body that would kill the head.

Most people voted "yes" in the poll. Where are you people?

Let's rephrase this as something morally identical: he ingested a spider alien and now a huge spider alien egg is growing on his back.

Pretty sure it'd be OK to kill it then. From the description it's more like that than a "second head".

this is a pradox.

my answer

its murder its not moral

yes I would chop that head off in a milisecond

I don't think this is morally identical. In your scenario, the answer would be too easy. In mine, they may be more conflicted with an answer, because they don't want to end human life.