Aha, been there done that! Regression is totally normal. I don't know why they do it, with my two I just put it down to too much confidence, or boredom with the new skill. If you are happy that he had got the control then whatever you do don't go back to nappies. Also do not show if it upsets you otherwise it becomes a control thing.
Start asking him frequently if he needs the toilet but don't beg or force him. When he has an accident just change him and mutter things along the lines of "Oh dear, poos go in the toilet not pants," and "it's really horrible when the wee goes down your legs, isn't it" but above all no emotion just curiosity! DD went on for quite a while and I cracked her by constantly telling friends that she had been a big grown up girl and had been using the toilet but I couldn't undertstand why she was a baby again. HTH