Quoting Megan Keyser:" The problem with the undies or naked idea is that he doesn't mind being soiled. Remember the story from ... [snip!] ... messes a little better and trying again in a couple months. With a new baby on the way, I'm going to have a lot on my plate. "
You need to encourage him to help clean up his own messes. When he smurfs himself you need to encourage him to help wipe his butt, so he those massive blow outs put him in the tub give him the wipes and let him try, he will see how hard it is to clean poop off of everything. Have him help wash the pants. If he whines and wants to do something else be firm and tell him we need to clean you now and you need to help with this we can not move on to the next thing until we are done here so now we have to clean. Of course you are helping too but you need to have him start taking responability of his body.
And you I would pair this method with the undies or naked and if you are confident that he is close and ready to pting then it should work. But if he's not ready then he may revert, and in that case you just need to give him a little more time, i wish they made better pullups for bigger kids.