My rat rocks back and forth and seems to stare into space a lot. Is he sick?

Not likely. There are many theories as to why some rats do this. Poor eyesight, nervousness, etc. Since most of the rats that do it are red or pink eyed, its most likely poor eyesight.
There is also a theory that since a rats eyes are on the side of its head and not to the front like humans, then the rat is trying to focus forward by rocking, so that it can view the object with both eyes, though one at a time. Someday I'll have to make a diagram for this one.