What you want is some vigorous activity that will provide aerobic activity for your dog in brief periods of time. The activity will help to keep him warm. If you have snow on the ground, the activity will probably be limited to a smaller area. One you can keep clear to provide good footing.
I made a toy Indy enjoys from a tether ball. He gets quite stimulated and plenty of exercise in 10 to 15 minute bursts. Doing this several times a day will give him plenty of exercise without overexposing him to the cold.
You can find directions for making it on another thread. Go to General Boxer Forums>The Boxer Ring>Toy Suggestions.
You can make sure your dog gets plenty of exercise without having to work up a sweat yourself, since he does most of the work. Indy can now "head" the ball three or four times before it swings away from him. If you prefer, there are "soft touch" tether balls available. I couldn't get one at the local Sports Authority, but plan on ordering one from the internet. He doesn't seem to mind the regular tether ball, but soft touch should make contact gentler. Don't know if it will rebound as well.
After 15 minutes of active play Indy who is very fit is breathing hard. He has to chase it, jump to butt it, change direction, so he is exercising every muscle group from head to toes while playing with it.
If you have any questions, you can e-mail me, or send a private message and I'll be glad to respond.
Regards, Tom