My Mom was a professional teacher. Her view was that you should go back to the beginning and do mastery & speed. For instance, start with your addition flash cards, and do the ones till he can do them as fast as you can put them down - with no mistakes. Then add the twos, etc.
Likewise, start with something like "Blend Phonics" and make sure he has mastered basic sounding out of words.
http://www.donpotter.net/education_page ... onics.html (the website is a favorite of Briva - I found it through her)
Also, reading "Why Johnny Can't Read" might give you a lot of help with diagnosing his Language Arts problems.
I'm guessing you'll find lots of holes in these areas.
A student who is failing in 8th grade has not mastered the basics - for whatever reason - and pressing ahead with 8th grade work when he doesn't get it will cause nothing but frustration.
Even if ALL he ever masters (book wise) is basic reading & basic math, he'll still be way better off this way than trying to work unsuccessfully on 8th grade stuff.
And, if he can do construction successfully & honorably as an adult, that's a great success!
If he is truly interested in construction, then perhaps you can make your exercises construction related (math problems related to board feet, or something like that)
This isn't MY advice, mind you - it's just what I remember from my Mother - whose advice I greatly respect.