L.,
My son is in the same boat. Maybe this will help.
We started putting him to bed earlier, after failing at moving bedtime later. We thought a later bedtime would lead to later mornings, but it was the opposite. He was up by 5:30 am on a consistent basis and we were losing it! Now I put him to bed at 7:15 or 7:30 p.m., and after a while it led to later wake-ups. Don't ask me why or how!
We also put a clock on his night stand and a piece of paper with the time he needs to stay in his room until (7 am). That took a bit of time, too, but it worked. Once he adjusted to the idea that even if he was awake, he wouldn't get to play, watch TV, or have mommy and daddy time, he sort of resigned to the whole thing and started sleeping longer.
Now he often sleeps until 7 am, which is awesome! And let me tell you, afternoons and evenings are much better now that we aren't in chronic meltdown mode from being overtired.
Best,
R.