I am the queen of broken bones and casts although I am not proud of that distinction at all.
When you're in a cast, your muscles can begin to atrophy and the extent depends on how long the limb has been immobilized.
Even without formal physical therapy it seems they would have given you strengthening exercises to get his muscles built back up. These can be as simple as toe pointing and flexing his toes pointing back towards him. Squat excercises are good as well. Standing on tip toe and down and repeating that.....
Those are things you can do at home and he should take it a little easy for a week or so at least so he doesn't pull a muscle or reinjure himself.
It's hard to keep little ones down. Thankfully they heal much faster than when we get older.
My guess is that if his leg still hurts him, he won't run and jump more than he can stand, but I would alert the preschool not to allow him to jump off of equipment or run around like a maniac for a bit just to make sure all is well.
My friends kid sprained his ankle and to be safe they put a cast on it thinking that would slow him down. It didn't. And when they took it off he ran around like nothing was ever injured and there were no complications.
I would say if he complains of pain or leg cramps, that's when you know for sure he's playing too hard. My friends kid couldn't wait to get his cast off and never even limped after so it all depends on the severity of the original injury and how long the cast was on.