Re: I am going to lose it!!!
I think you need some professional help to recover from this situation. Take a look at "Oh Crap! Potty Training". I think you'll find the advice you need there. She addresses exactly this issue in one of her chapters.
The key she mentions is to take away the reward he's getting for the behavior - namely, the reaction from you. So watch him like a hawk. When you discover him as you mentioned in your original post, you need to keep your cool, have him "help" you clean it up, and coldly say, "Poop on the carpet is bad. Poop goes in the potty. Next time, go to the potty."
And walk away.
Do not give him the pleasure of getting you riled up, do not tell him "it's okay", do not resort to punishment or rewards.