D.W.
Some days they just don't nap. And then they go right back to normal. I wouldn't worry about it too much. That's the beauty of having a routine rather than a schedule. Routines are flexible for baby's needs. There were days with both my little ones that they just didn't nap and the next day they were right back to their routine.
But the timing really depends on how long they are up between naps. I think the "norm" is somewhere around 3-4 hours after waking they'll go back down at you son's age.