Soups are nice because there's usually so much to prep - especially if you're adding vegetables. Also, have you tried fancy sandwiches? Those have an assembly component, in addition to (sometimes) asking you to prep stuff.
One of my absolute favorite cookbooks is A Super Upsetting Cookbook About Sandwiches. It is extremely inappropriate for children, so don't have him read the recipes directly. But it's great because it has a bunch of unique combinations, and he also breaks down exactly what makes a good sandwich (textural variety, etc.). Plus the Broccoli Classic absolutely slaps.
Recently elevated my sandwich game with this book... A Super Upsetting Cookbook About Sandwiches. Unbelievable recipes! https://www.amazon.com/dp/0804186413/ref=cm_sw_r_cp_apa_4yxxAbBSCP407
Check out A Super Upsetting Cookbook about Sandwiches; it's full of excellent out-of-the-box ideas. :)
Try reading this, it was from one of my favorite sandwich places in NYC
I just gave this book to my husband and he has become obsessed with it! I highly recommend it if you enjoy food 😋