It’s often about too little fiber, so you can start with cutting back on pellets (don’t take away all pellets unless you know he’s a good hay eater. If he doesn’t eat his hay he could starve, and that leads to a serious condition known as GI stasis very soon. Rabbits must always have food in their tummy). The problem can go away in just a couple of days. Obviously if he gets runny poop, poop with mucus in it and/or if he stops wanting food or becomes lethargic something worse is up.

This is a good article on the topic: https://rabbit.org/intermittent-soft-cecotropes-in-rabbits/