You could follow any sleep method exactly as described, but your hard work could be for nothing if something in your child's room interferes with his ability to sleep. One of us had a very similar experience with our child and the solution turned out to be as easy as removing a brightly lit alarm clock!
The next time your son goes to bed, try to sneak in before he usually wakes up. By spending time in his sleeping space, you can identify any environmental factors that may be causing him to wake up (like a blast of cool air from an overhead vent) or preventing him from going back to sleep (like a light shining into his crib). Pinpointing sleep inhibitors is a good first step to take if sleep training methods are not working.