I'm guessing it is the 2 to 1 switch (that's the only time we got EWs). There are basically 2 approaches to the switch:
1) keep 1st nap at the same A time every day, but gradually shorten the length of it; at the same time, move pm nap earlier (by the same amount you shorten nap 1). When nap 1 is <30 min, attempt to cut it out and just have 1 nap.
2) push morning A time later, allow your bub to sleep as long as they want in the am, then do a late pm catnap (can be a shorter A time than in the am, as you don't want them to take a full nap); eventually cut out pm catnap.
If he needs a longer A time before bed, then option #1 is generally better. If he doesn't and HATES being woken from the am nap, option #2 may be better. In both cases, you can offer an early bedtime if one/both of the naps is bad.
Whichever approach you choose - it's best to go gradually - so either cut 15 min off the am nap every 3-5 days or lengthen the am A time by 15 min every 3-5 days. It also is helpful to know about how much day sleep is "optimal" for your son and how much A time he needs (minimum) between his last nap and bedtime - that can help you plan out the day.
You may also want to head over to the support thread for some more ideas:
http://babywhispererforums.com/index.php?topic=158325.255hth
michelle