A.,
I would first check to see if it's an ear infection, but it's as likely just a growth spurt, a sudden dawning awareness that he misses things when he sleeps, teething, some subtle change in his crib or his room...Ha. Wasn't that helpful?
This was the age when we started letting our now 15-month old son cry it out and it was a lot tougher than even I'd imagined it would be. At the time we were saving money to buy a house and we lived in a one-bedroom apartment and it wasn't like there was anywhere to escape the sobbing of our baby. Man, I felt like a monster. In the end though what worked the best was done in part to soothe me. I decided I just couldn't let him cry for two hours without knowing he was okay, so we did the wait five minutes and go in rub his back and reassure him we were right outside the door. At first this seemed kind of counterproductive since his crying would ramp up as soon as we left him again. We'd wait a little while longer before going in again and going through the same routine - never picking him up or turning on the light. The first night when I tell you it took hours I am not kidding. At least two. I kept telling my husband the poor kid had to be dehydrated from all the tears. After about three really hellish nights it worked pretty well, though any changes mess bedtime up again temporarily. When we moved to our new (old) house there was a period where he was waking up in the middle of the night again.
I know this sounds sort of paranoid or overly dramatic, but he knows by now that he's winning. He's also probably waking up so early because he's overtired from staying up and tormenting you and your husband! Much as it pains you my advice would be once you've ruled out an actual physical ailment, go through your bedtime routine and let the sobbing begin! In the long run I think babies who can find their own ways to soothe themselves to sleep have much better long term sleep habits. A couple of mothers I know swear their infants who learned to sleep well young made the transition from crib to bed with much less drama.