All cat urine smells somewhat like ammonia as ammonia is a natural product of metabolism. However, some cats have urine that smells stronger than others. That can be due to illness or simple dehydration. Visiting the veterinarian will help determine the exact cause of the strong ammonia smell in Fluffy's urine.
Dehydration is the Most Common Cause
If your cat is not getting enough fluids, his urine will be more condensed and smell stronger of ammonia. If your cat's urine smells stronger than usual, a veterinarian can determine if the problem is dehydration. The solution is to make sure Fluffy always has clean water to drink. Place extra bowls of water in places where the cat can reach. Consider feeding Fluffy canned cat food as it contains water. Strong ammonia smell is also one of the more minor symptoms of lower urinary tract infection. Take him to the vet as quickly as possible if he's straining to urinate or has blood in his urine.