You might try one of the inserts made for those big potty seats. I understand there are a few different ones available. Balancing on the edge of an adult seat might seem pretty precarious to your son, especially while part of his focus is on having a bowel movement. A potty-training insert may make all the difference in his ability to relax – which really helps pooping. You might even look up online what's available, and have your son help you pick out the one he likes best. "Ownership" could make it that much more appealing.
There's also a specific approach that might help your son choose to be fully day trained. Check out http://www.rogerknapp.com/medical/pottytrainingrefusals.htm
You probably know that night dryness is not in most childrens' control. He will get there when his body matures enough in several different areas.