Oh Annette, I am the wrong person to talk to when it comes to A times, lol. I really didn't pay too much attention to them with DS2, it was more like what worked for the 3 of us together since I had DS1 to think about as well. Also, I have the world's worst memory and have pretty much already forgotten the details from that time!
I can tell you that we often ended up with a long A time between the am and pm nap. I was adament about the boys' pm naps being at the same time. So for the first A time I would follow his cues and it was usually pretty standard for his age. He would more often than not short nap, but I would not move his pm nap up at all, no matter what.
Nap times for us never fluxuated that much. I think his am nap was always between a certain time frame.
Whenever we dropped a nap we'd have long A times, but he always caught up in the end.
Keep working on it, but my best advice is to think about the whole family and do what works for not only DS, but you and DD too. Second children have to be a bit more adaptable I think, lol.