I did not read any of the other responses yet…
Absolutely let him do what he wants! He will not lose his intelligence; it will be there when he feels like using it. And I would assume that he most likely will want to when he’s older. A friend of mine who had a son older than mine at the time shared with me, to reassure me, how she was seeing her son changing before her eyes. As he matured mastery came into play and she could see her son who previously flitted from one thing to another, or worked hard at things only to not see them through, really caring about mastery for the sake of mastering. Well, I saw the exact same thing in my son as he got older! My very intelligent son (maybe not as intelligent as yours LOL) who also has ADHD really started to change around age 17. By age 18 he was on a real upward trajectory that has not stopped. He is 21 now, completely self directed and self-motivated, and very high achieving, except when it comes to laundry and cooking. Because (for other reasons) he did not take all the AP this and Honors that in high school, he didn’t like his college choices straight out of high school, and chose to go the community college route. He is now working on transferring and will surely have some great options, including “top-tier” universities. We always knew he COULD achieve most anything he wanted, but we weren’t sure for a long time if he would. Now it’s clear.
And another support for my position to let him do what he wants is our experience with our other son. We are unschoolers. In other words our kids never had to do anything academic-wise that they didn’t want to do. Our older son chose to go to high school (that’s why he didn’t do all the AP and Honors stuff because he was still new to formal education) after not doing anything much academic his previous years homeschooling, but our younger son, now 18, did not choose to go to school until a couple years ago when starting community college classes. So, think about it: this kid never took ANY classes, let alone hard classes, and he never did academics either except tutoring for his dyslexia when he was about 15, and self-studying math for about a year before starting college math. In our opinions, he’s doing great and will continue to do great. He’s gotten all A’s in the six college classes he’s taken, and is aiming in that direction for his current two classes. He can still choose to go straight to a four year school, or he can keep going and do the transfer thing. Either way, he will have plenty of options. He can still choose MIT or Yale even though that would be a tough one given the path we chose for him, but it’s not out of the question, but MIT and Yale would be tough for anyone! Point is, might some doors have almost closed, yeah, I guess, but are there a zillion other great ones? Yes. AND our kids are healthy and happy, had A LOT of fun growing up, and their self-esteems are great, not beaten down by expectations that were beyond their own wishes for themselves at the time.
Hope that helps.