My cat seems sick but I'm not sure what's wrong?

He has been eating more grass lately to throw up but only thick saliva is coming up, it had a light yellow tint, not sure if that's from eating grass right before or if it's a respitory infection. He isn't coughing or sneezing, but he is drinking more water than usual, not as hungry, having loose stools and peeing out of the box, not as active, and sleeping more in warm places like he is cold. Other than that there's nothing else to go on. I want to take him to the vet but these symptoms could be anything, nothing stands out as to what he has and if the vet can't find anything unusual then there's nothing I can do.

Take him to the vet. They will be able to take blood samples and do a more in depth exam.

He needs to get to the vet immediately.  He might have a urinary tract infection or a blockage.