Ditto to the first suggestion!
Also - people (including babies) tend to have 90 minute "alertness cycles" that will either interfere or help with getting them to sleep. From the moment you wake up, the cycle starts and halfway into it you are the most alert (45 minutes after waking) and then at the end of the cycle it is easiest to fall asleep. If you miss the "window" sometimes baby can catch a second wind and then get overtired and have trouble falling asleep the next time! Try keeping track of when he wakes up from his nap, and aim to have him getting ready for bed somewhere along the 90 minute cycles. For example, either 90 minutes or 180 or 270 (or 1.5 hours or 3 or 4.5... you get it).
The 90 Minute Baby Sleep Program is where I first heard of it, and it's been a life saver for us!
at 2:15 AM on Feb. 16, 2009