So he's trying out the phrase. If you (or someone else he listens to) say some word or phrase, expect him to try it, too.
What would you like your boy to say when something displeases him? If you want him to say, "I don't like taking a bath," then you say, "I don't like driving Dad's truck BECAUSE it's hard for me to get it in gear." Give a reason for your dislike. Maybe he'll pick up on the fact that there's a reason for disliking, not just a generalized attitude. That's very important.
You can say, "Mike, I made a mistake when I said I hated driving Daddy's truck. I meant just that I don't like it. 'Hate' was too strong a word for me to use, so I'm going to try not to use it any more. You can correct me if I do." (Kids love catching their parents in a bad word.) As far as the book is concerned, maybe the fictional dog feels strongly about his bath the way you strongly feel about Dad's truck.
If your son asks what the word "hate" is for, you can tell him what you believe is right. I'd probably say, "Hate is a very, VERY strong word, and it's more than dislike. Most of us don't hate very much, but we sometimes do dislike strongly! So let's use that word." You can add synonyms such as "abominate" or "despise" if your son is into words.