Advice to help my three year old deal with impending death of a pet?

Looks like we’re going to have to put my dog down. Probably very soon. Most likely will happen while my son is at school, and I’m looking for advice about how to prep him for coming home to no dog, how to say goodbye, etc. He knows the dog is very sick, but we haven’t broached the topic of death yet. Thanks guys, any feedback is appreciated.