So sorry to hear about your son's struggles. I have two pieces of advice that contradict each other: (1) a lot of this is normal; and (2) keep paying close attention and talking, because you're right that there's a very real risk here.
On 1: we noticed with DS that when he hit the teen years, he sort of slowly withdrew from a bunch of things he used to enjoy. Some of it was situational (e.g., the MS had IM basketball and rec leagues, none of that existed at the HS levels); some was world events (his freshman year was the Covid year). But I think in retrospect that a lot of it was developmental. DS is like me in that he spends a lot of time living in his head, and he likes to work things out in his own mind before he is willing to talk about it -- plus he's Mr. Silent-But-Deadly, who figured out at a very early age that if he smiled and was nice to everyone, he could basically do whatever he wanted (until he was busted). There's a reason we have the tropes about sullen teenages who lock themselves in their rooms with big "Keep Out" signs on the doors. And as parents, we need to give kids that age the space and privacy they need to handle all of the big physical and emotional and social changes they're dealing with -- and we need to support them in ways that show we understand that they are not the same kid they were at 5 or 8.
On 2: unfortunately, those same things that can be completely normal can also be a signal that something is seriously wrong. And it's really, really freaking hard to tell the difference. My own DS had a major breakdown sophomore year, I think largely from the whiplash of "normal to 18 months of Covid to normal," and particularly adjusting from freshman year no-expectations-minimal-academics to "ok, now here's all your AP classes." And despite all of my efforts, I could not tell the difference between "lazy and not wanting to do schoolwork" and "major depression" until he literally posted a note on his door that he could not face going to school.
I think all you can do is to work really hard to keep communicating, keep some sort of routines of things you guys do together, keep working to find things that interest him -- but, of course, do all that while giving him space and not pushing him to do things he doesn't want to do. I do love that fact that he is so into CAD; really, for kids that age, the most important thing is that they're really into something, and what that "something" is almost doesn't count (as long as it's not destructive). Is there some way you can build on that to open the communication doors more? That would show him that you see him for who he is, and gives him a chance to show off to you something that he's really good at. If you can meet him where he is, he might be a little more willing to do something like go on periodic dad-son hikes or whatever.
And if you figure out the magic balance, please let all of the rest of us know. . . .
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