Is he eating, drinking, urinating or defecating more or less than normal?
Cats who don't feel well usually don't want to eat. Some illnesses, however, can cause increased appetite, so don't ignore your suddenly ravenous cat. Increased thirst and urination may indicate kidney disease, diabetes or other illness.
Frequent, sudden attempts to urinate, especially if only small amounts are produced or if accompanied by signs of pain (including meowing or straining in the litterbox), may indicate a urinary tract infection or blockage. Inability to urinate is a life-threatening emergency that is all too common in cats, especially males. Here are a few other signs: