It could be the new litter. Sometimes little changes like that can cause them to do this. One of my chis, Rocky, did something similar recently, but with pee pads. We switched to disposable instead of washable ones (due to our washing machine breaking) and he started peeing around the house (yet he still pooped on the pads). Then when we switched back to washable ones, he went back to normal.
But whatever the reason is in your case, I would watch him like a hawk for a while until he's fully trained again. Don't let him wander around the house by himself, always have him in your sight. As soon as you see any sign that he needs to poop, take him to the litter and praise him if he does. Or like Susan said, put him in a pen with his litter when you can't fully watch him. When things like that happen, you have to basically go back to square one with training and treat them just like if they were a puppy again. It shouldn't take too long to get back to normal though.