One thing that worked for our son was a "live demonstration" with his dad. Gross I know, but I was desperate!
Also, I read somewhere that it takes 3 good BMs on the potty before they realize what muscles are needed to use to make it happen.
It was much easier with our daughter. I remember sitting in the potty with her (when I knew she had to go) and telling her to make a toot (pass gas) - that seemed to work good for her because they are similar muscle pushes. So you could try that approach. When you know that he doesn't necessarily have to poop, have a "tooting" contest! Again, dad's expertise is golden here. Later, when you're trying to get him to poop, ask him to toot and hold you hand to your ear as if you're listening. If he does toot on the potty - giggle and high-five and all that jazz and he'll want to do it again, and possibly all the tooting will lead to the big one.