You can try using vocal cues to guide him. For example, when you notice that he is pooping in his diaper, make the "ehhhhh" (grunting a little) sound, and when he pees, make the "ssssss" sound. This will help him associate those bodily functions with those sounds. So, eventually, when you put him on the potty, you can make those vocal cues, and he will eliminate himself on cue.
For this to work, you have to do some observation first. Watch his behavior to find out when he pees/poops regularly. Most little ones will go after waking up, a few minutes after eating, after popping of the nipple (if nursing), if you see the "poop" face, or the "pee" stare, after passing gas, etc. Those are the times when you should put him on the potty and then use the vocal cues.
I have done this since my little one was 3 weeks old. He is now 4 months old and pees/poops right on cue as soon as I put him on the potty. He rather enjoys using the potty and smiles after eliminating himself. :) I usually give him a minute or so and observe his behavior. If he looks like he's straining (tensing up, grunting, grabbing his knees or the potty, has a "serious" look on his face), then I know that he's just working one up. If he doesn't have to go, he lets me know by fussing a little or looking around the room (being bored) so I just take him off the potty and try another time.
So far, great success and saving lots of diapers! I read a book on this method and will gladly give you some information on it if you're interested.
Good luck!