DS has been at one nap a day for a couple months now. We let him lead the way, and just followed his cues. It took about a week, maybe 2 for him to find a good time to take a nice, long nap. Most days, his naps are 2-3 hours, but still, some days he only naps an hour or so.
What time does your DS wake up in the morning, and what time is his nap? We are pretty close to dropping the a.m. nap I think too.