E.G. is heavy into yoga lately, I think there is a lot you can do without stressing a foot too much. And a lot of it is a real workout. I'll ask him if he has any ideas for specifics.
I totally get the need for exercise re: mood, I "prescribed" a day at the pool yesterday because she'd had a weird week that was taken up by a badly-timed, too-far-away, not-very-active camp and she was going a bit stir crazy from lack of intense exercise.
If you hold Mo at the knees (rather than ankles) -- or figure out something he could hook his knees over -- he could probably do sit-ups.
Maybe you could even hold his ankles? Depends on where the break is I guess.
How much do you and Mr. B know about weight lifting? There is always the risk of dropping something on his foot, which would be bad, but Target and such have free weights that you can just buy and use at home.
There are also elastic band things you can buy (Target has these too) for various resistance exercises. You can definitely work up a sweat with those. (Thinking of arms here, mostly.)
He could also do knee pushups -- you know, the ones where your weight is on your knees rather than your feet.
Lots of exercises are from a prone or kneeling position -- I used to do "rovers" for x-country ski exercises, they're brutal:
Start on floor with knees under hips, hands under shoulders and back flat. Keeping head in line with spine, lift left leg to side no higher than hip height, foot flexed (as shown). Do 20 reps. Switch sides; repeat.
Technique tip Tighten abs and keep equal pressure on both hands while lifting leg to avoid tilting from side to side. The payoff? Serious saddlebag erasing.
• Works abs, hips, butt, legs
We had a bunch more variations of this too, I couldn't find anything with the whole set though.
Also laying down on your back and raising your legs until they're just a few inches off of the ground, holding as long as you can stand, then lowering them again. Sounds like nothing, but it's tough!