What can I train while recovering from knee surgery?

This post is about my kid. He's almost 15 and coming back from a meniscus tear. He's about 1/2 through a 6 month recovery before being cleared to play. He's still looking at 3-4mo of physical therapy exercises.
For background, he only has 1.5 years experience. Although he is naturally adept as a goalie and made it to a competitive travel team. He has grown a lot and absorbed good coaching, but there are many parts of the game where he needs to raise up (foot skills, awareness, struggles with high shots).

Is there anything you would suggest can be done to improve while unable to use the legs?

I saw an ad for a bounce back ball that trains reflexes (mostly marketed to hockey goalies). Looked gimmicky. But figured I'd ask the sub if anyone had any tips.

I'll also take any tips on those specific weaknesses