My cat is pooping blood and he seems to be in pain. He won't lay down, he just sits/half stands.?

My cat hasn't been eating well for a couple months so we switched him to a wet food. The first week he did just fine and ate a ton! He slept all day yesterday and hardly moved today. There was blood in his poop tonight and he seems to be in pain. He is sitting/half standing in his favorite bed but he wont just lay down and doesn't purr when we pet him. I'm taking him to the doctor in the morning, but does anyone have suggestions for tonight?