I agree with AnnieBrooke - get a medical check up. To me, you actually sound depressed, which can sap your energy.
But I think your concerns are well-founded. If he's not doing anything boys like to do - sports, biking, hiking, etc., it's really time to change that. His childhood is slipping away. At this point in his life, as Annie said, he might prefer to do activities with friends - so this is something you can support without having actually engage in the physical activities. You can take him and a couple buddies to places where they can explore, bike ride, etc.
Have you looked into Cub Scouts? If there are children who want to join but their parents can't afford the fees, they work with that and often can give you a discount or waive the fees. Same thing for most kid's soccer/baseball. If you truly can't find room in your budget for these activities, youth groups will usually work with you.
For one thing, get a checkup. You might have anemia or even a thyroid condition. A whole lot of women are borderline anemic, and it really can knock you down to be low on iron.
Does he have any friends he can do bike rides with and play outdoors?