Within the Adobe stable, I would recommend two: Photoshop, or Photoshop Elements. They are similar, but PS has a few more "bells and whistles," can work in CMYK (for commercial printing, i.e. ink on paper by an offset printer), and a few more items, like Layer Sets (PSE can edit them, but not create them), and the price is vastly different. With either, I would look into getting a tablet, like a Wacom.
Outside of Adobe, Corel's Painter is a good choice too. Once, it did a lot more "natural media" work, than PS, though some of that functionality has been added to PS, version to version. I work with both programs, and would not be without either one, but that is just me. Both programs, PS and Painter, will work in PSD format (PS's native format), though there are a few little differences.
Now, and not "painting," or "sketching" per se, Adobe Illustrator is a great Vector Art program, but from "sketching," there will be a bit of a "learning curve."
Good luck, and I would post any specific questions to the Photoshop, Photoshop Elements, or Illustrator product forums. There, you will get some great input, especially as this forum is for discussion OF the forums, themselves.
Photoshop and Illustrator.
A drawing tablet is a must. Wacom is one of the best known brands http://www.wacom.com/en/products/intuos/small.aspx
Have a think about a monthly (or even 12 month) membership to the Adobe Creative Cloud (starting at $30/month)
Suggest you start with a free 30 day trial for each product you are think of. See what suits your fancy.
Also look into the educational edition. THey are the same product, but just much cheaper if your son can qualify.
Illustrator CS6 and this book:
The purpose of having this book is that little Jonnie can practice everything from the book with example files that come in a CD with the book.
As suggested by Curt, it is a good idea to start with the trial version that gives you 30 days free of charge and then you can buy an academic license as little Jonnie qualifies for it if he is still attending a school and/or college.