Reddit reviews Julian Is a Mermaid

Yay, so glad that you and your son are enjoying Red! If you liked that one I suspect you'll like Neither as well :)

I am somewhat familiar with Are You A Boy or Are You a Girl?, but it's not one of my favorites - it's fine, but it's just a bit dry, you know? I think it's tough to write a more factual book that's actually engaging for kids. Jamie is Jamie is slightly more successful at that (similar concept, the main character's gender is never specified and they demonstrate that there's no such thing as gendered toys, activities, etc). And there are a lot more books about gender non-conforming kids than trans kids - some good ones are Jacob's New Dress, Julian is a Mermaid, Annie's Plaid Shirt, Sparkle Boy, and Teddy's Favorite Toy. There's also the book Worm Loves Worm, which is probably intended as a metaphor for same-sex marriage, but arguably works more broadly as well (since the whole point is the worms don't have genders!).

...and I could go on about other books showing different family structures, but I'll stop there for now :P Let me know if you'd like any more recs, I'm always more than happy to talk about picture books!