J. - that's so annoying! I'm sure it's impacting your sleep too...
Is the room totally dark & quiet? We've discovered they sleep so much better, longer, etc, with total dark.
Your son is a lot younger than our kids, but we've had a situation with each of our three where total dark cured our problems.
The older one was waking up midway through the night - like 3 a.m. and not able to fall back to sleep. The middle girl was not able to fall sleep at all. The baby (5 at the time, so not a baby compared to yours) was waking up way, way early and waking us all up. In all three cases, darkness was the key.
Apparently we cycle through sleep stages, and get "near awake" a couple times a night. At the "near awake" point, light or noise can stimulate a child to come fully awake and stay that way.
Anyway, try making sure your son doesn't get any of the morning dawn. If that's already the case or doesn't help, then I have NO idea what to tell you.