Good evening - I'm Dr. Sara. I'm sorry to hear that you're having a rough night with Otis. I know that it can be very scary when they aren't feeling well.
It's not unusual at all for cats to posture and strain to produce only very small amounts of feces whenever they are having a bout of diarrhea. This is what happens when the large intestine is inflamed - we call it colitis. Sometimes they will also pass blood or mucus with their loose stools. It's very difficult to tell what exactly set off an episode of diarrhea in most pets. Often vets will treat symptomatically with probiotics and anti-diarrheal medications without ever getting a firm diagnosis. It could be viral, bacterial, dietary (like he ate something that upset his system), or parasitic, among other things. Diarrhea is a very nonspecific sign, meaning lots of different diseases could cause it.
You are in a particularly tough spot because he is diabetic - it's important that diabetic pets continue to eat and get their insulin on schedule. In general when my diabetic patients are not eating, I recommend that they be examined to find out why before things get too out of control. In your case this may involve a trip to a local veterinary ER - your regular vet should be able to refer you to the nearest ER, in most cases this information is available on your regular vet's after hours answering machine message. Diabetic patients cannot cope with illnesses as well as healthy pets, so a simple viral bout of diarrhea that your average cat would get over on their own could become life threatening for a diabetic patient.
Another thing to watch - be sure that he has been urinating normally. Sometimes it can be very difficult to tell if a cat is posturing to urinate or to defecate because they both look very similar. Cats can get urinary blockages that cause them to strain and push repeatedly to urinate, but they are unable. If he has not produced any urine in the past 6-8 hours, I would be concerned for urinary blockage and recommend immediate emergency care.
Please keep in mind that I'm advising you based on your description and my education and experience. This is no substitute for an in-person exam by a veterinarian, especially with a diabetic kitty.
I sure hope this helps - please let me know if you have any other specific questions that I can answer for you.