If you think it's just harmless fun when your puppy chases his tail, think again. It could indicate a neurological problem or a serious case of boredom, and it can escalate into more destructive activities like self-chewing. Stopping the behavior while he's still young is key.
Take your pooch to the vet, just to rule out any neurological issues. Sometimes puppies and dogs end up doing compulsive things like chasing their tails because something is wrong upstairs, so it's best to rule that out right away.
Give the little guy plenty of attention and exercise him every day. Puppies often have more energy than grownup dogs, and when they don't get enough stimulation, they can try things like tail-chasing to burn off some steam.
Teach him a few tricks, like sitting and lying down. When he starts to chase his tail, distract him by giving the command to do a trick, and when he does, reward him. After praising him, give him something to play with, like a rawhide, that will hold his attention and help him entertain himself.