Reddit mentions of Rosie and Crayon

Your son is really lucky to have you there to help him process all of this - as you say, it's not just about the fish but about his entire 13 years of loss and trauma. I hope he's seeing a good trauma therapist, and that you can let them know about all this stuff that's coming up for him right now.

I know 13yo is not exactly picture book age, but there are some beautiful picture books about loss and death that have helped me get through some rough times even as an adult. I highly recommend Lifetimes (about death as part of the natural cycle of life), Always Remember (about how the memories of those who are gone live on), and A Stone for Sascha (wordless picture book in which a girl grieving the loss of her dog finds comfort in realizing the vastness of time and space). A couple others that might be especially meaningful for him are Rosie & Crayon and The Rough Patch, both of which include main characters who are so traumatized by the loss of a beloved pet that they feel as if their life has lost purpose and meaning, but slowly learn how to rejoin the world and love again.