If he is OT, I would bring it back to the 3 hour mark for a few days and see if that gets him back on track. For the PM nap, if you know it will take him awhile to go due to begin OT then lay him down even earlier so that by the time he does fall asleep it will be right on time, iykwim. Not too early though or he may not be tired enough to actually go to sleep. I know, it's such a fine balance....crazy sometimes, huh?!
-Melissa