What could be going on with my toddlers potty training success?

My son turned three March 27, and for several months I have been potty training him we started off with him stark naked, then moved to training underwear, then to pullups. He did great at first but now for no apparent reason has begun to soil himself. I feel like breaking in tears as he won't communicate with me what he wants. Some children communicate that they want to wear their pullups, some underwear he don't communicate with me at all. It seems like he doesn't pee/poop only when he gets to run around naked but when he is clothed he doesn't care if he goes in his underwear or pullups. I can tell he is frustrated with potty training right now, and wonder if I am at fault like maybe I pushed him too hard. What could be going on? Should I back off and let him go it alone or keep encouraging (which leads to a power struggle)?