Hmm, maybe you could challenge his math skills a little by doing some orienteering work (requires a bit of Trigonometry), something like geocaching, but with compasses and treasure maps instead.

If he's into cars, how about combining a work activity with a go-cart trip, or maybe get him into r/c racing and help him build a cool 4x4 racer?

These are just some ideas. I'm sure if you put your mind to it you can come up with things he would be drawn to even though there is some physical challenge to it. The point is to be patient, encouraging, genuinely excited about it yourself, to the point where you will go do it without him a couple times and come back excited and pumped up so your enthusiasm will be contagious to him. When you find that mechanism that becomes irresistable to him, then you will be able to draw him into wanting to try, and that should be the impetus he needs to start building his own drive mechanism.

I had a nephew that was a lot like how you describe your son. He was in therapy, and got put on drugs, and went through a lot until he grew up. At one point I reached out to him by offering to teach him how to shoot and how to hunt. He was captivated, and for a time even willing to help clean the guns, long before we started shooting them. Unfortunately his mother was uncomfortable with the idea, so it ended, and he went on to therapy and a lot of what I considered unnecessary anxiety. Now he is legally limited as to getting to handle firearms (one of the disqualifiers on the purchasing form). Because he was "diagnosed" and treated by a clinician.

I firmly believe you love your kid. I encourage you to be do all you can to help make them successful, even if it doesn't always feel good.

Again, Good luck.