The Brain Pickings website is a great resource. You can sign up for the weekly newsletter (comes out on Sundays), or just go on the site and search for "Children's Books". It's where we've found some of the more interesting and even thrilling picture books.
http://www.brainpickings.org/?s=children%27s+books
Also, try different lists and categories on Goodreads to inspire your son to think outside the box a bit. For example, picture books about math. We've enjoyed a bunch of these:
https://www.goodreads.com/list/show/71503.Children_s_Picture_Books_on_Mathematics#13633558
Speaking of thinking outside the box, the library is definitely the best place to do that. We tend to have 20+ books out at a time, and they're a mix of books that DCs have picked and books that I've picked. Great way to experiment with different things and to let them experience the joy of discovering a great read (and the low stakes of picking a "dud" that you can just return the following week.)