I can’t think of any books that I know of, sorry. You might be able to find some videos on YouTube that he could watch, or just finding digital art that he could use as reference.
Aeden Peterson (Andrew Peterson’s son, I don’t know if you’ve heard of him) does some really nice digital art. Also, Chalice (who is on Story Embers) is really good at it. You could probably tag her and ask if she has any advice.
Forgiven. Loved. Creative.