What is wrong with my cat???

My cat is 10 years old, and a diagnosed diabetic. His sugar has been well-controlled for about 2-3 years now. Recently (the past two months), he started eating voraciously, and yet he is losing weight. His diabetes is not the problem, because his count is so good, (checking once a week now) he has not received any insulin in a few months. We took him to the vet for a complete bloodwork, but so far she has found nothing. She checked for a thyroid problem twice, with negative results. She now wants to do a bile acid test to see if anything is wrong with his liver. Oh, and he is constantly scratching and grooming, pulling out tufts of hair (but never irritating the skin) At first we thought he was grieving (his little brother died about four months ago), but this has gone on way too long to just be emotional, and we are careful to give him much love and support. We are desperate - we can't lose another kittie! Does anyone have any ideas??? Thanks in advance!!