Blood in loose stools

My rescue cat takes Viralys for control of Herpes which helped the symptoms but now has stools which are formed at first but then it changes to loose consistency with bright red blood. His stools are every other day. Other than that he has lots of energy and seems healthy. I feed him chicken once a day and have Purina One sensitive, urinary tract, and hairball formula mix available to him all the time. What can cause the abnormal stools and what can I do to correct this before I take him to the vet?

What do you mean you feed him chicken once a day? You can't just give chicken, it's not balanced. A little treat is fine, but not as a meal. If the supplement is causing diarrhea, you should stop it. Blood does happen in diarrhea sometimes. It's not an emergency unless it's going on for more than a few days, your cat is vomiting or lethargic, or not eating. 

http://www.foodfurlife.com/my-cat-has-diarrhea---what-do-i-do.html

How old is he?

Thanks for your replies.I don't know how old Charlie is exactly.A couple of years ago the vet thought he may be around 4 or 5 yrs.old by the amount of tartar on his teeth.I decided to take away one food item at a time to see if that was the culprit. The food that caused Charlies digestive problems turned out to be Temptations treats. I leave Purina One Sensitive System dry food out for Aurora and Charlie. My vet said the moitened and shredded baked chicken breast or turkey is good. I only give him approximately 1-2 Tbsps once a day and a few pieces as a snack. The problem now is his stools are hard and small. I'm going to try Laxatone because he has a tendency to get hairballs from grooming Aurora daily. If that doesn't work the vet said I could try 1/4th tsp of Miralax in enough water to dissolve it and use it over the poultry I give him. Other than the digestive problems Charlie is extremely lovable, happy, and seems to have enough energy. I'm so grateful the bleeding has stopped !