V.B.
I would not go back to diapers at this point. You're probably going to have to come up with some kind of reward system that will incent him to want to go. This happened with my daughter. She had NO accidents for about 4 months and then all of a sudden decided she didn't want to go poop in the potty anymore (this was much harder for her to do in the beginning as well). She would actually tell us that she didn't want to stop playing to go. Anyway, she's over it now, but we had to step up the rewards. I took her to the ice cream store and let her rent a movie. That was just if she would poop in the potty for one day (we had to give her some immediate gratification so that she would be able to connect the reward to the potty). Then, we started a sticker chart and when she filled that up, we would go back to the movie store or go to the dollar store and pick out a toy or something along those lines. Now, she doesn't even ask for the stickers anymore...it's just routine again. I noticed you have a new baby in the house. I think that had something to do with my daughter's regression too even though she didn't start training until after my son was born and had it down. There came a point when I think she felt like she wasn't getting enough attention and that's when this all started, so it was a control thing and an attention getter for her.
Try some different rewards and see if you can get him started again. Also, you may want to make him help clean himself up. My daughter didn't like how long that took (especially if she had to help) and decided it was faster to just go in the potty and be done with it!