TB-Still use the 2 hr rule: If he wakes up from his afternoon nap and it's 2 pm, put him back down by 4 pm for a third nap. If it's closer to 5pm for his third nap, pretend it's bedtime and put him down for the night. I know you would think he will wake up way too early the next morning, but he'll actually sleep longer and possibly past 6-6:30 am, which then will move your am nap back later and poosibly the day. The reason he isn't sleeping through the night right now may be two-fold. One, many babies are not ready to NOT eat for 12 hrs until they are 9 months old! Two, if he's overly tired (ie: going a long stretch in the afternoon without a nap) he will not sleep as deeply and soundly and will more easily be aroused at night.
Check out the age appropraite section in Healthy Sleep Habits, Happy Child, by Dr. Marc Weissbluth (it sounds as if that's the book you may have already read). He will explain more in depth exactly the trouble you are having. And yes, this is very common (and very frustrating to deal with) at this age.
Your Pediatrician should also be a good source of advice for how to navigate this tricky time! Remember, on-line advice should be taken with a grain of salt! Trust your own physician on problems like this, that's what they're there to do. Good luck and happy sleeping!
Dr. S. McNulty, Mia Bella Pediatrics