If he’s still healing from the neuter, you want to watch for blood, redness, swelling. Take a hand mirror, lay it on the floor, offer him a treat so he stands up to reach it, and check out his stitched area. If all there is is dirty poop, he may just have pooped softly or sat in cecals due to recovery from the surgery.
How is he today? If his site looks good, not discolored or swollen, you can take a very soft cloth like a hanky, dampen it, and gently touch the area to clean off the exterior of it. Do not rub him, because you don’t want to upset the incision site. What’s possible is that he will have cleaned himself there by now, at least of excess poo, and look better. If he looks at all unwell, call your vet asap.
If you wait a few days more, and he is fine but still dirty, in about a week you can sprinkle corn starch on his bum. It soaks up moisture, dries the poo, and comes off more easily. Again, he will clean it himself, but it’s safe for him to lick.