The doctor has determined that he's got RDS, which is basically underdeveloped lungs. The lungs don't expand properly as he breathes because the lining and structure didn't have a chance to fully grow while he was still in the womb. This is a situation particularly prevalent in premature babies since the lungs are one of the last parts to really develop.
He's been put on a ventilator which is injecting a mix of 60% oxygen into his lungs. He's also receiving medicine to stimulate the lung development and some relaxants to ensure he isn't in pain or straining too much. He just needs some extra time to catch up.
Although the situation is fairly serious, he's been responding well to treatments. He's also at a very healthy body weight and otherwise seems to be well. But it's very hard to watch him there, as he's hooked up to machines and we can't do anything else to help him.
On the plus side, Kit's doing very well. She was discharged from the hospital today in good condition.
And thanks, everyone, for the kind words. Even though I haven't had time to respond to all the comments on my entries, they are appreciated!