You'll find that babies are very adaptable and your little one will adapt to the routine your childminder has.
My son went to a childminder when he was a year old but he was still bfeeding day and night (for naps and comfort as well as milk).
He just went without milk during the time I was away and when I picked him up, he'd have a catch up feed.
As your little boy is 9 months and will still need milk, I'd maybe start giving him a tt cup of milk (not for naps though, just to make sure he'll take it).
My little boy would only nap after nursing or being pushed in pram and CM was happy for him to nap in his buggy and he napped when he was tired (more easier for her than for me!).
If you don't think you'll have enough expressed milk, then yes, I would get him on a formula feed now (out of cup) and send formula to the childminder (I think he's a bit young to go without milk for a full day, but they do say older babies will just adapt and make up for the milk when he sees you, but you might not want to spend your nights being a milking machine :-)
I don't think you have to do anything else to get him ready - I did nothing for mine, other than the taster sessions beforehand (I'd leave him with childminder for 60-90 minutes twice a week, a few weeks before).
He will be fine and will get used to things at his CM, and your CM will help him settle in. I don't think you've messed up at all, and I think you have plenty of time to get your boy used to formula in a cup, which is probably the only thing you want to sort out before he attends CM.
I know how you feel about leaving him, but he will be fine (honest!)