My 7 mos old generally is up for 2.5 hours between naps. I just started that last week. He was on a great routine of 1.5 hour long naps (morning and afternoon) and 2 hr long awake periods. He suddenly started having short afternoon naps and fighting me when it was time to go down for his nap.
We did have problems with 45 min naps around 3-4 months old but they disappeared only to reappear now!!! So, currently his a.m nap is 1hr long then he is up for 2.5 hrs and then his afternoon nap is anywhere from 20mins to 1.5 hrs long. Usually it is 1.5 hours. He still has a catnap around 5:30-6:00 but he doesn't go to bed until 8:30. He will then sleep about 10.5 hours at night.
When DS wakes early from a nap I try to put him back to sleep. The last few days I have given him a soother (which he has never used before so I am not sure if it is good or not ... he does have a cold though
) and he will go back to sleep for another hour. When he is awake and showing tired signs I try and keep him up for the full 2.5 hours and it doesn't seem to mess with his nap time.
Sounds like your DS is a great night sleeper. He also has a good am nap so maybe all he needs is a short 45 min nap in the afternoon ... I found that when I tweak my lo's routine (usually awake time) his naps seem to fix themselves.
Hope this helps