Unsure he’s pooping

I’ve got two cats, my youngest (Ozzy) has his own litter box but often prefers to use his brother’s. So it’s hard to know for sure since the one box gets used more.

Since about Friday I’ve started to wonder if he’s pooping. I can accurately tell 100% of the time the difference between their pee. Poop is a different story, but there are often noticeable differences. So yesterday afternoon I put Ozzy in his room (a good sized bedroom with windows and everything a cat could ever want) where his litter box is to monitor and know for sure. We’re about 18 hours in and still no poop. He doesn’t really like wet food so I gave him a Churu last night to up his liquid intake.

My question is, at what point do I worry? If I knew for sure he hadn’t pooped all weekend, I’d be considering emergency vet and definitely calling the vet in the morning. Can I wait, am I freaking out for no reason? Is there something else I can give him to help him poop?