Good luck tomorrow. Be prepared for a lot of crying but the child should fall asleep eventually and you should notice that each day the crying time gets shorter.
I have had a few parents tell me that their nap time routine is to rock their child to sleep or feed them a bottle in their arms before bed. I have had a couple of interviews ask me if I will rock their baby to sleep and cuddle them until they are asleep. I always go with honesty and say that I try to teach them to self soothe because I cannot rock or soothe 4-5 children simultaneously at nap time. I know I have lost clients based on this. Sure, I could have lied to these parents but it was a red flag that our parenting/provider methods were not going to mesh. I always try to be upfront and I really don't believe children should be rocked to sleep. I never did it with my own two and obviously thought CIO was best for them.
I just could not imagine myself rocking a 12 year old on my lap some day. lol
I hope your transition goes well. It won't be pretty because the guy is going to be very angry and confused as to why you are suddenly leaving him all alone but you are not traumatizing him and he will get the new routine in time.