I feel like perhaps in an effort to make sure he felt "normal" growing up, I've under-challenged him and now he is ill equipped to move forward.
It sounds like you've done a great job of educating him and giving him a wonderful childhood. All other things aside, you've given him one thing in particular that pays off in more ways than can be counted in terms of a happy future and self-confidence - you've been a caring, attentive and involved parent. That is, jmo, about 99.999% of the equation toward helping children become whoever they were meant to be.
And that's the other side of the equation - who are our children meant to be? I think it's easy to buy into the idea that because a child is highly gifted intellectually they should also naturally be uber-motivated to achieve amazing things. Really what most of us want for our children in life is to be happy. We can't superimpose a personality type onto a child simply because they are uber-capable. It's also not easy to know at 15 what type of motivation a child is going to have at 17, at 21, at 30, or at 50. I wouldn't over-think it now in terms of worrying that you have somehow not created a situation in which your son has learned motivation or worry that a perceived lack of challenge will automatically mean he won't find his motivation and be extremely successful in life. You'll find accounts of adults who are intellectually gifted who blame lack of challenge in their childhood education for lack of motivation and success as adults, but I suspect you can find many many examples of highly gifted children who grew up to be successful adults - there is simply a lot more to the equation of happiness and success in adulthood than what happened in childhood, and personality and resilience playing a huge part. And that's for kids who didn't have the best of circumstances - from what you've written, your son has had challenge and thrived in his education so far.
What to add? He likes sports. Is he playing on any teams? Being able to work as part of a team is a great life skill, and sports also provides a lot of fun, both "now" while he's still young, but also as an adult if he finds a sport that he can continue to participate in. Add in the health benefits too
If he likes science, you might look for an internship with a local company. If you haven't got any ideas or connections yourself, it might be worth asking if the school district has any type of mentorship program and see if he could participate in that.
If he likes computers, he might be able to find a way to use his computer skills as a volunteer.
I'd also add, I have three teens - one is 17 and a junior in high school. He also likes science and is a high-IQ kid. He has never seemed to be extremely motivated about school and future career. He never seemed to be incredibly motivated about anything until he found his favorite passion (which is a hobby, and he sees it as a hobby). The thing is - he had his motivation, it just didn't show on the surface and it's just taking some time and maturing to come out in a form that looks like motivation to an outsider peeking in. Even though I've spent a lot of time with my kids and feel like I know them really well, the things that are at the root of who they are becoming aren't right at the surface for me to understand, they are being revealed slowly in their own language as time goes by. My 15 year is very different than my 17 year old - she's always been extremely motivated to do well in school, worries about getting As, wants to start practicing yesterday for her SATs, wants to know what she has to do to get into a good college etc. Yet none of that really defines the essence of who she is in terms of how motivated she is or how successful she'll be later in life, or even how successful she'll be on her SATs compared to my older ds - it's just a different personality in terms of how she approaches life. As long as our kids don't have to live in a world where they feel the world expects more of them than they can deliver, or feel that they aren't worthy or are unloved, or that the world holds no hope for a happy future etc... I think they will be fine. BUT... that's just my opinion
Best wishes,
polarbear
ps - the other thought I meant to add - if you haven't already, ask your ds what he thinks and what he'd like to add in or how he'd like to change things up. He may have some great ideas. He may not have any ideas at the moment, but they may start to come out as time goes by.
