If your dog's anal glands fail to properly express, they may actually become impacted, making your dog very uncomfortable. Watch for these signs:

• Your dog begins scooting or dragging his rear across the floor.
• Your dog keeps licking or chewing near his rectum.
• Your dog's stools have become soft and mushy.
• You'll likely notice a foul or "fishy" odor coming from your dog's rear.

If you are noticing one or more of these signs, it may be a good idea to take your dog in for a check up with his vet. The dogs anal glands may actually need to be manually expressed, a job best left to the professionals.

It may be from the food your feeding him. Dog food with to much cereal can cause a gland problem.

My 2 Huskies are notorious for this same problem. My female would always walk & throw her rear end on the ground with a loud yelp. Always licking at it. I had no idea what is was. One night I noticed blood all over coming from her rectum and rushed her to Emergency ER to find out one of her glands had ruptured. A very scarey experience. Please take your dog to a Vet and have them emptied.  It is a nasty smelling brown liquid, that dogs use as a calling card. This is why they always greet one another with a sniff of the butt. Like our shaking hands. Albeit.. a gross greeting in the eyes of the human.

Good luck.