A Funny Story and a Plea for Help!

Crumpet keeps me on my toes, that's all I can say...
The potty training is coming along better than I thought it would. Phew. Crumpet will now pee on the potty, no problem, and even goes by himself without telling me. Ah, self-sufficiency. I never thought I'd see it. (Yes, my bathroom is gross right now...) But he still has issues with pooping. (Isn't it amazing what moms feel is okay to talk about and publish? Yuck.) Today we had a little friend of his visiting, and they were having some trouble playing well together. At one point, I went into Crumpet's room, and he was huddled in the corner. I asked what he was doing, and he told me he was hiding from his friend for a while. I asked if he was really going to the bathroom, and he said no. So I left him to be alone for a little while. Eventually, he came out of the room and told me "That was a really bad poop, Mom." I said, "AACK, in your underwear???!!!" But no. He had put on a pull-up and put his pants back on (backwards!) so he could hide in the corner and fill his pants! How he reached the pull-ups, I don't know. I didn't even know he could put on his own pants because he always tells me it's impossible. So obviously, he is capable of many more things than I realize (I did suspect this...). He is very sneaky and clever, and thank goodness he put on that pull-up!
Here's my trouble, though. The little fib about the fact that he wasn't going to the bathroom, when in fact he was. He has done this many times in the last few days, always as a way to get what he wants when he's afraid I'll say no. It's always pretty entertaining and clever. But how do I stop it before it is no longer funny, but a full blown problem with lying? We've talked about telling the truth, and I try to make most of the things he wants to do possible, so he won't feel the need to hide from me, but it's not working. Any ideas?