Check out.. Www.petplace.com They are super at answering questions. But I have two cats and when there tails are down there searching for something or their upset for some reason. I check the litter box or offer a treat or just spend some time brushing them!
They might be scared
It all depends he may not like his food or he may want to be pet or... Ect ect
There's a possibility that the cat's tail could be broke and you should take him/her to the Vet to get it checked out because it could be something extremely serious
If the cats tail is going between its vack legs a little it means it is probably scared.