This sounds like an anal sac infection, and you should take him to the vet just in case he needs antibiotic therapy or to have it drained. One thing you can do to prevent this in the future is increase the amount of fiber in your dog's diet. That helps create a stool that puts enough pressure on the sac to express it as the dog defecates. Orijen 6 has 3% crude fiber. This product is a supplement that has 5%:
http://www.onlynaturalpet.com/product_detail.aspx?item_guid=C3203D7F-F765-4333-8D36-DDB9E3F615B8&click=6875&mf=2
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"If you talk to the animals they will talk with you and you will know each other. If you do not talk to them you will not know them and what you do not know, you will fear. What one fears, one destroys." - Chief Dan George
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