Vanity publishing is always an option, though I have a whole diatribe about the expectation vs. reality of vanity- and self-publishing.
You might try going to a bookstore to see what other whimsical science books have been published in the past year and reach out to those publishers. (And it doesn’t have to be a publisher that already has other geode books. On the contrary, you want to find one that has sciencey books, but a geode deficit.)
While you may find many examples in the library, it only helps if the books are recent. A store can tell you or show you which new ones are selling now, which will give you some info on the market, which you can then fold into your pitch
For the record, one speed bump you may hit is that you already have art for the book. Most publishers like to have a say—if not outright control—over who does the art for their books. So, if that’s a dealbreaker for you, just be prepared. If not, consider pitching just the text of his book and see if more doors open for you.