Well, I will start by saying that I also have had great sleepers (knocking on wood), and I believe in keeping 2 naps as long as possible. However, your son does sound typical of transitioning to one nap. I have an 18 month old who is doing that right now.
If I were to guess, I would say that you are putting him down to late for the nap. Even if he doesn't need two naps, you are basically keeping him up 2-3 extra hours in the morning, and now longer since he is getting up earlier. Watch his signals closely and get him down before he gets overtired. My older daughter napped 12-3 when she went to one nap. I would do lunch at 11 and get him down ASAP. You can always move it later if he doesn't sleep. But I bet if you put him down at 11:30-12:00 he will sleep three hours.
You need to try to establish a time for several days (like 4-5 days) for him to adjust to consistency. If that doesn't work after several days then go the opposite direction and try to put him down later at 2-3. But inmy experience, one-year olds start at 12 noon, and by age three they are going down closer to 2-3pm.
It takes awhile but it will shake out. My oldest napped for 2-3 hours until she was four.