My cat might be dehydrated?

My cat, Scout, got bitten by a dog about a week ago. It got infected so I took him to the vet. They put a drainage tube where the bite was to make it heal. Scout now has a cone over his head (to make it so he wont scratch out the tube) He was given antibiotics and pain killers. I'm worried because he is not interested in drinking water. I ha ve been giving him soft food and putting some added water into it. Im just worried that he isn't getting enough water. Suggestions?