Here's a marvelously comprehensive website discussing family issues, with an overview of just about everything possible to do with sleep: http://www.med.umich.edu/yourchild/topics/sleep.htm
I've also heard that children who do not receive enough natural daylight or excercise during the day may have a reduced ability to sleep. Television late in the day also can contribute to this. The light from the screen is in the blue "daylight" end of the spectrum, and it interferes with the body's ability to self-regulate and produce melatonin, a natural brain hormone that induces sleepiness at night.
Hang in there. Some people, children and adult alike, just don't sleep easily or well, and your efforts may need to be continued for awhile yet. It's not their fault (I've never been a good sleeper, no matter what I do.) My grandson had a similar pattern, and at around 2-plus, his folks introduced a little jar with just a few marbles/pebbles in it. He'd get to choose a favorite breakfast item if he still had some increasing number of marbles in it when he woke up (starting with just one).
So every time he got out of bed, he was handed the jar and had to take a marble out of it. He could visibly watch his chances of bacon, pancakes or french toast diminish every time he popped out of bed, and he quickly began to see this as something that was within his control. His goals were easy to start (important for a young child) and increased every few days. After maybe two weeks, the getting up pattern was broken. He would still be given special breakfasts and much appreciation for his new ability to control his own behavior.
Another thing that might help is to find out whether anxieties about the dark, shadows, or physical separation from you are contributing. He might do well with a nightlight – perhaps let him choose a nice one (avoid light in the blue-green ranges, which can interfere with sleep).
Good luck, I think you're doing the right approach in general, and it may simply be a matter of time before your son realizes he will stay in bed and come up with his own strategies for dealing with the dark, restlessness, longing for parents, etc.