I think I can relate to his rehab. I once popped both hamstrings at both ends -- had to have both reattached at the groin and behind the knee. Was bedridden for 3+ months in a right angle cast -- cast from my toes to my hips with a bar cemented between my ankles to keep my legs 90 deg apart and prevent any lower-body motion.
Once his bones are sufficintly healed to start rehab, the first step will be manual range of motion work. Meaning a trainer will forcibly push his legs in various directions to extend his hips, knees, and ankles. There will be a lot of resistance to break through. At keast daily, build to 3x daily as he can take it. That month or so will be rough.
And mentally taxing because he'll be in a wheelchair that whole time. Once the casts come off, it's really hard to remain patient and stay off your feet, and continue to trust the process.
Then they can do the assisted walks, light weight work, and the like, that build him back toward independence.
Two things that will help him: he'll get a ton of in-person support from family and the team, and he'll know Sasso recovered from worse.