Dog's head is moving side to side, loss of coordination suddenly..what's wrong?

I have a six year old greyhound. An hour ago, he started moving his head back and forth, like he was watching something waved in front of him. I put him in his crate and gave him honey, thinking it could be low blood sugar. That didn't help. He ate all of his food and drank all the water I gave him. I left the room, came back, and he had blown out and urinated in his crate. I took him outside, where he peed a little, but the head weaving continued. I took his temperature five minutes after coming back inside, and it was right at 103. Usually, he's a pro at taking the stairs, and trots right up them. But he couldn't take a single step on the way back inside, and actually tripped when he tried. I had to carry him up the stairs. He isn't panting or trembling, and when I hold his head still, his eyes continue that back and forth movement. ...What's happening to my dog?