Board Thread:S,R,&G/@comment-198.53.100.217-20150524112736/@comment-9294464-20160830134433

Assuming they even changed his mouth movements from the Japanese edition, which would mean they know what he is saying

Dubs never edit lip flaps, so you can count that possibility out.

For reference, here's an excerpt from that scene in the LN:

Kirito's piercing glance looked at Aincrad, for a moment he was downcast. His lips moved, whispering a name, but she couldn't hear it.

When Kirito raised his head vigorously, his face had his usual dauntless smile back on again. He spread his wings wide, aiming for the sky in front.

"Ok— Let's go!!" So, beyond that description, nobody  knows what he's (canonically) supposed to be saying there.