LOL...kick him out! I was a homebody, too. Nothing was better than curling up in a warm spot and reading a good book. My mom would kick me out and tell me to get some exercise. Usually I would pedal to the library and read there or walk to a friend's house, so we both got what we wanted!
On the other hand, is there any way the two of you could take a "study break" together and go riding or walking your property? Any friends who can come over and play with him?
One thing I do with my sons is tell them they need to play outside and if they choose to remain inside, they can do chores. That usually gets them outside pretty fast. They have the best imaginations and, along with the neighbor boys, have the best time making stuff up. My backyard has gone from being a WWI trench warzone, to home of the coolest fort ever, to trampoline central, to play structure fun to the practice ring for nerf swords and battle axes. I've watched them play Star Wars, Pokemon, Knight's Tale and so many other fun, odd, boyish things. Just last Sunday they were playing in the back yard (a mere 1/4 acre) and they would shout "POOR LIFE CHOICE" every time someone made a bad move. Not quite what their father and I meant with the phrase "poor life choice" but at least they're listening, right?
It's hard, dear mama, but with 10 acres, your son should be able to find at least 1 hour's worth of fun physical activity.
Good luck!
PS: My boys are 17 and 13, if it that makes a difference at all, and their friends are in the same age range.