Did your little one have a sanitation cut? Were his glands expressed? My first thought would be irritation from being shaved down there... Sometime's it just feels funny and they'll scoot because of that. It doesn't necessarily mean they're in pain.
If his glands were expressed, take him to the vet and have them checked. If they weren't expressed enough (or properly) that could be irritating as well. In the future, refrain from having groomers do that & take him to the vet for it instead. It's really quick and for the vet it's very simple. Hope his bum feels better
