Summary: Follow the interests of the child and get books related to those; make math fun first, including games.
What is a good book series he can pick up on? Something to get invested in and want to read, rather than the forced 20 minutes before bed every night.
My first choice would be to follow the interests of your child, and get books related to those topics. It could be sea adventures, dinosaurs, pirates, dragons, etc. What does your child like to listen to when you read aloud? Any favorite games? What do friends of you child like to read? An interesting topic will drive the interest in reading for kids. A recommendation from a friend or example of peers can do the same too. (The same is true for adults).
My second choice would be to take a look at a few lists of "best children's books". This is not to be confused with "required reading" assigned by some teachers or schools. These vary widely across teacher and schools. Depending on a particular case, these could be quite boring, or not age-appropriate, both in the experience of me years ago and the kids nowadays.
Try a few books from the library. Read aloud to the child the beginning of the book. See what catches the child's interest. Many libraries are parts of large networks of libraries and you can get get lots of books for free by requesting them using an online library network catalog and interlibrary loan. The books can often be renewed online. You can even specify a particular edition that you like, if it is available in the network, for example, hardcover, or with illustrations that the child likes. Don't push "good books" if they are boring to your child - these are highly individual preferences.
Also, I would look for either books or book series, rather that only for book series. For example, Twenty Thousand Leagues Under the Sea by Jules Verne, Charlotte’s Web by E.B. White, A Light in the Attic by Shel Silverstein are great individual books, and they are not parts of book series. Below are a few examples of such "best children's books" lists that have some books that we found to spark the kids' interests (very subjective choice, YMMV).
Best Children's Literature of All Time: http://www.clcawards.org/Best-Children-s-Literature-of-All-Time.html
100 Great Children’s Books | 100 Years | The New York Public Library: https://www.nypl.org/childrens100
50 Books All Kids Should Read Before They're 12: https://www.commonsensemedia.org/lists/50-books-all-kids-should-read-before-theyre-12
The Ultimate Backseat Bookshelf - 100 Must-Reads For Kids 9-14 - NPR: https://www.npr.org/2013/08/05/207315023/the-ultimate-backseat-bookshelf-100-must-reads-for-kids-9-14 . Some of these are okay for 7 yo despite the "9-14" in the title.
I'm going to try and go beyond the math homework and do some dedicated
math practice every night. It's just tough finding time as he has an
older brother and we also have a 1-year-old girl, both requiring
attention and both major distractions for him.
As for math homework and math in general, I would start by making the process of learning math more fun and interesting for the child (including games, etc). There are plenty of fun math books and math games. The games can be also fun for parents and siblings. So the older brother can become a valued partner, rather than a distraction. Put your main effort into this - and the homework will often follow.
In my experience, it is more important that the child learns math than whether the homework is done. From my own experience and that of others, the child who knows math will often do the math homework, even a boring one, more effortlessly and quickly. A life-long love of math, the roots of which are planted now, is very valuable in many fields bot now and in the future.
We and others had good experience with these fun resources:
Books by Jane Kats, especially the MouseMatics series. For example:
MouseMatics 7-8. Learning Math the Fun Way (Volume 6): https://www.amazon.com/MouseMatics-7-8-Learning-Math-Fun/dp/154264092X/
Games by the Brainy Band ( https://www.amazon.com/stores/node/17346211011 ), such as:
The Brainy Band Frui10: https://www.amazon.com/The-Brainy-Band-TBB002-Frui10/dp/B01N8VBKGJ
The Brainy Band HurriCount: https://www.amazon.com/The-Brainy-Band-BB-1-HurriCount/dp/B01MA49R5K
The Brainy Band Splittissimo: https://www.amazon.com/The-Brainy-Band-Splittissimo/dp/B07CNKWS6C