I second the Fr. Lovasik books, even if they might have a tendency to wear out (though not, I’d think, any more than other paperbacks)—they’re absolutely great content, and you can get them very inexpensively—enough to replace regularly or give multiple copies!

Here are some “mainstream” children’s titles that I think would be perfect for a Catholic school library. The saint books of Tomie de Paola would be a beautiful addition—if you are limited by funds or availability, I’d choose the ones about St. Francis (Poor Man of Assisi), St. Patrick, Mary Mother of Jesus, and Guadalupe. There are other good ones, though, so that’s my own personal pick. Also, The Clown of God, and Now One Foot, Now the Other teach love of God and love of fellow man, respectively. I haven’t read his books about the Miracles of Jesus, Bible Stories, or other books specifically about Jesus, but I think they’d be worth investigating.

The Hobbit by J.R.R. Tolkien is appropriate for most middle-school students, and some other writings of his would be a good introduction—something with Farmer Giles of Ham, for example.

Wow, this is getting lengthy. Some more titles, Catholic and mainstream:
Saint George and the Dragon by Margaret Hodges
Know Your Mass by Fr. Demetrius Manousos
Beauty by Robin McKinley (more for older kids)
Catholic Tales for Boys and Girls by Caryll Houselander
books by Inos Biffi
Because I Love You and other books by Max Lucado
The Giving Tree by Shel Silverstein
The Princess and The Kiss; The Squire and the Scroll by Jennie Bishop
The True Princess by Angela Elwell Hunt

One more note: I discovered the best fairy tales and Greek mythology in my Catholic school library as a child. I recommend the d’Aulaires and Andrew Lang.

My kids usually started with the vision books about age 9 or 10 - maybe 4th or 5th grade.  Also consider the Mary Fabyan Windeatt saint books and the series of saints books from Daughters of St. Paul (Encounter the Saints) - both of those are for a little younger readers (2nd - 3rd grade).  Also, the Bethlehem Books for quality fiction.

Re the homeschooling situation and your boy with ADD, have you tried doing something like, do the first 3 questions and you can do X (whatever activity he loves to do for 5 min.), then the next 3 questions, etc.  Or tie an amount of screen time to how quickly (not to rush, to stay focused) he gets it done—like finish 10 ques. worksheet in 15 min. (or however long you think it should take) and you can have 15 min. of screen time.  The reward has to be whatever is motivating for him, that’s the key as we all know! 
Eventually you can go for the intrinsic rewards learning can bring, but to get him going, you may have to try this type of thing.  This kind of jump-starting children with extrinsic rewards to do their work is so hard for me (even as a teacher!), as when I was a student I got a great deal out of the resultant learning, grades, comments, discussions, etc.—it was rewarding in and of itself However, many children just don’t, at least for certain periods in their learning years certainly.  My heart is with you. 
Also, re the medicating, it likely will be completely shocking how much he will be able to focus and if he has handwriting problems, you will barely recognize his beautiful writing withinn a half hour of him taking the med., if you eventually decide to do that.  However, having said that, I suggest going for the non-stimulant type of meds, as my son lost weight on the stimulating kind.  (Also, he had other issues ongoing, so it wasn’t a cut and dried thing with him.) 
Finally, you may want to see if he has sleep apnea—easy to get tested at home.  My son had sleep apnea and after he had a tonsillectomy/adenoidectomy it helped his symptoms some.
God bless you and your family.  One day at a time!

I know this isn’t exactly homeschooling advice related (I’m just starting with my son this Fall, so I don’t feel too equipped to give advice on that), but I wanted to share anyway.
My husband was held back in grade school because of his inattentiveness.  It wasn’t until he was an adult that he discovered how much of a role his diet played in his abilities.  He has severely restricted dairy, wheat, and several other things, and does take regular vitamin supplements, and it has made a HUGE difference!  I can’t say if it would help your son, but I wanted to mention it anyway.
And, in encouragement, despite his difficulties my husband learned to cope - he now has 2 Masters and a Doctorate with a GPA over 3.8!