My goodness, that is awfully early!
First, you already know that you need a solid, consistent routine that is followed every day. Consistency is key - LOs really thrive on that. Don't let HIM change the routine, that's your job and the sitter's job. My suggestion is to leave him in the crib until the desired wake time, settling as needed of course. Then go about the day on the desired routine. It might make for a couple of really awful days with a lot of complaints, but he will get the idea that this is how things will be. Just make sure that the routine is something that will work for him, that he can do, so that you don't set him up to fail.
Also, I think your naps need some adjusting. You're right to want to extend that 2nd nap, keep on that one! But after the third one, that's a long A time for a 5.5 mo. at nearly 3 hours. He's probably overtired by bedtime.