I think there are two big things to bear in mind: (1) at his age, he's going to change quickly and so will his sleep needs, for instance, he's about due to drop that catnap in all likelihood, (2) children are immensely good at adapting to different circumstances, so he might do things there that he would never do at home.
I also think there's not much you can do to prepare *him*, I wouldn't change anything you're doing at home, for instance, though I might consider introducing a lovey, if he hasn't got one, it might make the transition easier for him. I would think about what you can reasonably ask/expect of the CM, and I'd start talking to her (I assume her!) about it earlier rather than later. If you tell her that you're worried about his sleep, she will probably be sympathetic and try to help. The really hard bit is likely to be your mental and emotional struggles with it, so try to think your way around it a bit now. It is unlikely that he will sleep the way he does at home, so get ready to hear that from her at the end of the day without flipping out (if you can).
And I think the short settling in periods sound like a great idea. Maybe he can nap on the way there in his buggy if he needs to? If you have a sling, you could bring that, he might nap in there for you, or for her even. A friend who is a CM does 'wear' one of the babies she minds and he loves it. The settling in period is really about him and you getting comfortable with the environment and getting to know the CM and the other children, it's not much of an indicator for how his day will be.
((Hugs)) it's a tough transition, it really is. It's not just you, or your LO. Try not to fret about it too much.
