Thank you for rescuing this little baby! I'm sure he is frustratingly cute.

He is probably claiming his territory like some of the other PTers said.

Is your porch screened in? I have my three cats in a screened in porch and it works good. If you do have a screened in porch you could move his belongings out there, and if he is pooping directly on the porch you can put a litter box where he is & he should use it.

Although.. Take this by NO OFFENSE. But do you keep missing him do that? If so, he might be trying to go out but he cant hold it since he's a baby kitten & can't hold it like other cats can.

Sorry, I tried my best to help.