Chronic Constipation in a german shepard

Question
My brother has an 8 yr old shepard, typically in good health.    
For the past few months he has had difficulty with his bowel movements.  When he passes one, they resemble marbles.  Taking him to 2 vets, external exams didn't show anything.  After taking him to a third vet, once the dog was asleep (in order to perform a BE exam, the exam didn't show anything but this vet decided to open him up anyway to double check.   
A "twisted" bowel (I'm assuming lower) was discovered.  It was removed and the bowel was reattached.  
A few days following, very loose bloody stools were noted.  A few days after this, the constipation returned.  The dog was put on Miralax.  This produced some loose stool but the dog really didn't improve.
The Miralax was stopped.  The dog is now yelping when trying to make a bowel movement and the rectal area appears swollen.  My brother lives alone and works 10-12 hour days.  The dog stays in his house and does not relieve himself until he is let out at night.  Could this be the cause or does it sound like a tumor?

Answer
Hi!
If the rectal area is really swollen, it is possible that your dog has a perineal hernia. But of course I am only assuming, since I can't see your dog.

Other reasons for decreased bowel movement may be spinal problems. Have they checked the spine?

Also what is the food you are giving your dog? You may want to consider giving him something that has more fibre in it like Hills w/d.

Kind Regards,
Caglar Kondu, DVM