my male, butters, started spraying in our laundry room at just that age and while he didn't poop, we promptly got him fixed!! please try this first and give him a chance! this is the #1 reason cats end up at the shelter and it's often a stress or health-related issue that's correctable w/a little understanding. i know it's no fun, but try getting him fixed right away and see if that doesn't fix (forgive the inadvertent pun!) problem!! good luck!!
Probably because of the pregnancy. He sounds like he's trying to assert his territory. I'd take him in to be neutered right away (like this week) before this becomes his standard habit.