My boys always had disturbed sleep schedules during big milestones--ie. crawling, walking, vocab burst etc. My assumption is that your little guy is working through something like that as well. You didn't really say what his nap schedule is like, it might be he's working from a 2 --> 1 nap a day, and or it could be he's not napping enough.
What was his schedule like before he started waking early? Honestly, whenever my boys did the waking early thing, it was because they were over tired and/or teething. Generally, tho' it was because they didn't nap well that day. I moved their bedtime back 1-2 hrs and then they would sleep an additional two hours. I always can expect foot steps at 5:30am when we had a late night before....
At this age most children need anywhere from 12-14 hrs of total sleep. My oldest literally would take a 3.5 hr nap and then sleep 12 hours at night. My youngest will only take 1.5hr nap and barely sleep 11 hours at night. So it could be your little guy is on the lower threshold for the number hours of sleep he needs, but anything below the 12hr mark, I would guess its more of the overtired not sleeping issue.
Good Luck--sorry its such an exhausting phase....I'm there with you, but its the we're done with napping, but Mommy is pregnant and needs her nap. Isn't going too smoothly.