Potty training can be really scary for children. I have read in several places that children feel like their BMs are part of their bodies. Therefore they feel like they are losing something when they go on the potty but that in a diaper keeps it with them. I know that it sounds strange, but it was true with my little boy too. He would go tee tee on the potty but not poo poo.
I was able to potty train my little one by 2 1/2 by letting him pick out underwear of a favorite character. Elmo worked for us. We also read lots of books about going potty.
Good Luck! Don't give up! He will get it!