The vast majority of kids go down to one nap at or near 15 months if they are sleeping well through the night and all. So he may be closer to going down to one longer nap instead of the two shorter naps anyway. If it were my boy, I'd wait until he starts so he continues to get the good rest he needs now. It will likely be an adjustment to sleep in the same room as other kids anyway. When he starts, he may be distracted by all that's going on anyway in the morning. The baby room is not real conducive to sleep for a 15 month old because there is constant activity as the kids take naps, eat, etc. at different times. For the two hour nap in the toddler room, lights are darkened, usually classical/sleepy music is playing and such to drown out stimulants. So he may adjust to the one nap better than you think. A lot of kids adjust really well within a week or two. That said, I do home day care now (after previously being a teacher at Chesterbrook Academy day care preschool) and one of the kids is 18 months and still needs that short nap in the morning along with the afternoon nap. I give her the nap early so she still sleeps a good long rest in the afternoon with everyone else when the house is quiet to work toward that one long afternoon nap. She also doesn't sleep real well at night, so I think that's why she still needs the 2 naps at 18 months. Her morning one is down to 30 minutes though. Hope this helps with your decisions. Best wishes to you both with the transition!