There are lots of great children's books dealing with the topic. Most of them also have sections at the end which give information to the parents or small exercises that I sometimes find pretty helpful and effective. Unfortunately, I'm not an English native speaker so I can't recommend any specific book, but you might search for one which fits the interests of your son and then read it together and talk about the story while gently trying to let your son identify with the main character and get an access to his own feelings. (and of course wait for other slps to reply ♥️)