Congratulations on custody! : )
I like your "dummies" idea.
I don't know your son, and I know that at 17 a young person can be mentally well into adulthood, or in contrast - still wanting to play with dolls or trucks. Which makes it a little tough to recommend something from a distance.
For an (almost) adult student who feels restless, videos might be easier to take than a textbook.
I found this with a quick google (don't know how good it is or isn't, but it looks promising)
http://alison.com/search/result/?q=English+Grammar. It is "college" but I've never taken a college English overview course that didn't start right back with "what is a noun?"
Also, with our foreign language study we've had remarkable success finding good stuff on Youtube - believe it or not. If you get an outline of grammar topics like this one: (
http://www.sinclair.edu/centers/tlc/pub ... basics.pdf) I would predict that you can search youtube for a video on each topic, and compile an entire video course. They have lots of stuff for adults - like ESL, or remedial students - so you shouldn't have any trouble finding stuff age-appropriate (I was just looking this up the other day for an ESL friend, so I know they've got a good selection)