An unfortunate thing that happened was kids not getting to be kids when everyone got sue happy.
Kids ...... even those my kids ages (30+) spend/spent many many hours inside on computers and video games. They never really learned how to be outside.
Not saying it's true of your kiddo just in general kids have lost that "there's nothing to do inside lets go ride bikes" way of thinking.
There is a lot to be said for the benefits of making something with ones hands and hard work. That sense of accomplishment and the expenditure of energy cannot be replaced with pills.
Is he willing to help with projects? Maybe get him to help with a pallet project like making a potting bench or outdoor seating?
Tell him you really need his help to do xyz project and DEFINITELY get his input on what he thinks of this or that.
Just brainstorming sneaky ways to get that young man some nice sunshine and outdoor time.