@boomerang,
Almost this exact thing happened to sozlet's BFF a couple of years ago. Just as summer break started, she broke her leg.
She had a special thing for her cast that allowed her to go in the pool. Something like this, though I can't swear to the brand.
http://www.waterproofcast.com/
If I remember right, she couldn't do that at first, but she could do it for much of the time she had a cast on. (It was a long time with cast/ crutches for her, I think 10 weeks?)
Also don't discount things that include just walking as opposed to running -- Kay (BFF) got a fair amount of exercise by just hobbling around on her crutches. It was a lot of work, which is both positive and negative. (Tiring, but she also is someone who needs a lot of exercise.)
She also had physical therapy, which was exercise -- weights etc. I don't know all of the details of that, I think it was through the hospital and was covered by insurance.
Some of it she did on-site, some things she was supposed to do at home.
She had a full-leg cast -- she couldn't move her knee at all. So that was really limiting. If Mo can move his knee, there should be more options available to him. I know a few grown-ups who messed up their feet lately, they were still quite mobile. Usually they had a cast and then a boot (more options with a boot).
I agree with ehBeth that you'll probably get a lot more info on this from the doc.
How is Mo doing? Kay found the whole thing really traumatic (there were some circumstances around it that made it extra-bad, though), her mom assumed she'd want friends around but for the first couple of weeks she'd only see sozlet.
Good luck to Mo + hope the recovery is smooth and quick...