He's up at 7:00am, because that's all the sleep he needs. I think you will be hard-pressed to get him to sleep more than he needs. One of my boys gets up BY 6:00am most days. That's just how he is, I've tried everything. He just doesn't need more sleep! As you said -- I just had to come to grips with it. My suggestions...
Option 1: Keep him up later at night, so he'll sleep later in the morning.
Option 2: Allow him to get-up at 7:00am, with strict instructions to play quitely until 7:30am. For example: read in bed, read in his room, play with quiet toys in his room, watch TV quietly, do not wake up his sister, etc.