I'm sorry about your cat. I would suggest scheduling it while your son's at school, and talking to him in plain language to explain what happened. Don't say that the cat is now living at a farm or other euphemisms, but instead be as clear as you can be to explain that the cat's body stopped working and that the cat's not coming home. Explain that that's what happens to all living things when they get old and/or really really sick.
There is a book, Little Cat, Big Cat which you could get from the library to read to your son, but it's very emotional and when I read it after our cat died, made me cry during story time.