Hi E., I have a brilliant slacker as well (now 18 and a freshman at a co-op engineering school on Merit Scholarships in a 5 year masters program, home Friday, yay!).
When he was 5ish he went through an intolerable miserable phase where I was just at my wits end with this nasty child. I took him to a psychologist because clearly I was failing him.
She tested him in every imaginable way and shed a lot of light on what it was going to take to keep this genius thriving.
And she was right of course. He needs CONSTANT intellectual stimulation, needs to be learning, needs to be challenged, REALLY challenged to keep him motivated. So I did just that. At school, I buried him, as advanced classes as I could find, throughout HS he also took classes at our local community college, he needs adult interaction, adult conversation. He also was involved in a zillion different other activites, took summer classes, and worked.
When his life is slow, easy, he is a bear.
My next son is directly the opposite. When he feels pressure he shuts down.
We have a very relaxed happy household, so I like to think by virtue of being a member of it, my oldest sees the value of enjoying life and family....but mostly he wants to be smarter more successful then everyone he meets. Whereas the next one wants to relax and enjoy life, well, least he seems to have a better balance.
The things I did to engage him that his therapist suggested will not work for your fella since he's already older.
But if your guy is like my, the trick is to give him MORE, not LESS expectations.
Like your guy, he would cop out of things that were just not that sexy, ENGLISH for one thing, simple chores around the house. And I would remind him, with humor, hmmm, I thought you were BETTER than THAT, I thought you were the smartest kid in the school, and you you can't even figure out how to vring your clothes upstairs? Interesting....
Enjoy him, it's been a trip for me to share things seemingly age inappropriate with him only to find out he ALREADY has formed an opinion on the subject.
Cheer him on, keep him engaged!
:)