Hi, I am Dr. Peter and would like to help.
1- Where is the clear sac material located?
He is defecating and the feces is encased in a clear sac like material, right?
What you are describing is "mucus." The colon is irritated and inflamed (colitis) and as a result produces mucus. Common causes for colitis include infections, parasites, inflammation, etc. I would recommend to consider having him checked by your veterinarian. In the meantime, change his diet to a bland diet of rice and boneless chicken. In addition, give him 1 teaspoon of plain yogurt twice a day for 2 weeks. If he has not been dewormed in over a year, consider getting him dewormed.
If you have additional questions I will be glad to answer them.