I let my cat drink saline
Why would you left your cat drink saline solution?
He's been seeming to want it for about a month.
Pets such as cats need fresh water to drink and not saline solution. Saline solution is a mixture of salt and water and you should not assume it would be healthy for your cat to drink it since the salt can affect your cat's kidneys. You should be a responsible pet owner and stop letting your cat drink saline and instead give it only fresh water.
How bad can this thing be? It's just a shot of salt water.
Salt water is unhealthy for cat's as it can harm their kidneys and it is dehydrating. It definitely is a bad thing and it's your job to take care of your cat properly. Perhaps you could call you vet and discuss things that are good and bad for your cat to consume.