I think the island, or whatever it is, was telling him that he had to go find that plane (and basically give the island Boone) or he'd lose his ability to walk. That basically the island was saying 'I gave you your legs back, and I can take them away if you don't do as I tell you to do)

or something.

I did read somewhere that Dom aka Charlie said that Charlie was going be a darker character than her currently seems to be, and I wonder if he might be doing things but not realising it - like he's possesed or something.

Someone on TWoP had the same idea that Ethan was maybe trying to protect the baby. And I thought I was so original.