Well, some signs of a cat being diabetic are drinking a lot of water and urinating a lot. You may want to have him checked (I happened to read your post on today's cat about him drinking a lot of water).
We just happened to find out she had diabetes because during her checkup her glucose levels came back abnormal and the vet asked me if she was drinking a lot of water. She was actually trying to drink out of the toilet and hopping into the bathtub to drink.