P.M.
Three thoughts come to mind:
1. He's not really "too old" yet not to be completely trained. Pressure at this point sometimes drives kids to withhold and create serious health consequences for themselves– constipation, painful movements, anal fissures, and encopresis.
2. If he'll sit on the potty with a diaper, I've heard that some moms have success with cutting a hole in the diaper so the child actually poops in the potty. After several tries, toddlers sometimes realize they can skip the diaper.
3. Here's a very specific process for empowering the child to train when ready: http://www.rogerknapp.com/medical/pottytrainingrefusals.htm. It looks like a great system, though I have not had to use it with any littles, so my opinion is tinged by the reported success of other moms.
You might find this site informative and supportive: http://www.parentingscience.com/potty-training-tips.html. It discusses the various methods of potty training, their advantages and disadvantages. A very thorough resource.
Wishing you and your son the best.