How do you give a heads up to family when you’re about to say something you know they won’t like?

I’m about to break the news to my son that we won’t be getting a second dog and I know he will be very disappointed. He’s 4 and I don’t want to do this without warning him, but it feels weird not to and I worry that I’m making this up. I usually say, “I have something to tell you that I don’t think you’re going to like.” Then he just asks why and I explain gently. Am I making this up? This is a small example but I’m an adult now and have to tell my parents stuff they don’t like from time to time. Sometimes it’s a double whammy because I want to hide the truth to save feelings but I’m forced to admit the truth. How do you do this?