Thanks for the suggestions!
To the people who commented on treats:
The treats I'm feeding him are these:
. They're very small, and we don't give him many - maybe he gets 3 per week. I'm not convinced that an amount that tiny could interfere with digestion, but I'm definitely no expert. (First-time pet owner here!)
Milo is hard to 'train'. I can't always use affection to reward him because he has moods where he doesn't want to be touched, and he doesn't always care when we praise or scold him (on the other hand, when *he's* in the mood for it, he loves to be picked up, cuddle, sit on our shoulders, sleep curled up with us, "groom" us, etc!). He's very willful, he's an "I do what I want" cat! I've even resorted to the spray bottle at times, and yes he will continue scratching the screen doors while my husband or I spray him and say "NO!!" His desire to have fun or explore seems to outweigh his desire to please us. We're making progress though! But because of that, I'm very hesitant to take away a method of rewarding him that works.
Morquinn - yes that does make sense, and I'd never thought of that before - that a cat might move his tail and muscles around the base of the tail to help him or her eliminate. Milo does have a tail, and I had the vet confirm his tail is normal (because when we first got him, he would chase his tail and bite it - I was worried he didn't realize it was part of him!), so I don't think that's his issue. I hope you're able to solve your tail-less kitty pooping problem!
Here's what I decided to do:
After reading Auntie Crazy's post, I've been gradually increasing his wet food and decreasing his kibble, so he should be at 1 can and 1/4 cup kibble in a few days. If he continues to have the occasional poop-sticking-out problem, I'll try pumpkin puree with no sugar added like Catloverami suggested, and if we still have problems I'll switch brands of treats, then food.
We're only able to consistently put out wet food for him twice per day, so I'm hesitant to take away the kibble entirely as long as he's growing. I'm kind of at a loss as to how to balance "feed him as much wet(raw is too much for me currently, maybe in the future) food as possible" with "always have food out for a cat under one year", and "kittens like to eat small, frequent meals".