Not sure if you are looking for advice or a perspective, but I'll try to offer little of the former and a lot of the latter (most of which some social scientist will probably come along and tell me I'm wrong on). So, in other words, what you are about to read has no redeeming value.
Anyway, from the perspective of a 39 year old male. First, there have always been a bunch of slugs amoungst my gender. However, it does appear that tendency is becoming, as you so, epedimic, or at the very least, growing. I don't think this should really be surprising. If you look at the way I was raised verses the way you were probably raised (and the way my children are being raised), I had a lot more freedom. When I was 3 years old, I could go out of my house and walk aroun the block by myself. There is no way in hell I would let me kids do that now. When I was 7, I would take my fishing pole and walk down through the gulley and thru the woods to the creek about 1/2 a mile away and go fishing for hours. Do you think that happens? How much do you see 7 year olds without their parents around? Or, even the same age, we'd ride our bikes a mile away and play pick up football and baseball. At night in the summer we'd get together and play kick the can well into the dark. When I was 14, we played baseball in other towns and parents drove us. At 16, we drove ourselves and parents mostly stayed home. In highschool I worked 30 hours a week my senior year. When I was 16 and 17, I got up early and went duck hunting, often by myself, before school
That stuff doesn't happen now. Sports are predominately played in an organized fashion. Kids aren't allowed to venture out of the house by themselves without parents watching. So, the kids stay inside and watch TV and get lazy. Now, obviously both boys and girls are subject to this, but girls have always semed to be more inside creatures than little boys. They have their dolls and their tea parties, or have a friend (which now has to be an established fucking play date) over and do thing with them. Boys have to be stimulated in different ways, so they migrate over to video games, where they proceed to sit on their asses and play them for hours (and I've done this too).
So, the boys get conditioned to be layabouts, and as laws of physics dictate, bodies in motion tend to stay in motion, bodies at rest tend to stay at rest.
There are some male bodies in motion in your age group. In a couple of months if you can't get him in motion, I'd suggest it is time to think about finding one.