Please only informed opinions about my diabetic cat -- could you, please?

I have a 7-year-old Bengal who has chronic virus in his eyes and respiratory system that he picked up when he was seriously ill as a kitten. He's managed to go along fairly happily all his life with good care. In January, he was diagnosed as diabetic, and he is getting 1.5 units of vetsulin twice daily and a diabetic (high protein with fiber added) diet. His blood glucose tests are good. This is a cat that has weighed 18 pounds when he was a little overweight, and he is now down to 12 pounds (very bony looking). He constantly begs for food, but then will eat only a few bites. He doesn't much like the diet dry food (combination of Purina DM and Purina high-fiber), but will try to steal my rescued kitten's kitten chow (but even that, he'll only snack at if he manages to get some). He has very loose stools (this is what the additional fiber was supposed to help with), and I haven't been able to put any weight on him, even with giving him a tablespoon of high protein canned food he likes several times a day. I would really appreciate any serious advice. PLEASE don't tell me to take him to the vet. He has been to the vet too much (and she is a very good vet). Short of having him undergo major expensive tests like full body scans, they can't think of anything else.