Geez sounds like a party.
I broke my neck 3 yrs ago, 5 cracks over 3 vertebrae C5-7, had brief left arm paralysis. No surgery but it has affected my body, last year in august "complex tear of the medial posterior meniscus".
I'm 57 yo, skiing the best of my life and dying to get to 1 legitimate ball at -38, (aka get all 6 at -35) and now it's really hitting me that this life long goal I have might not be realized. Bummer
But still, I train 4x a week a time the gym. Can't run anymore but I'm still in Great shape. A very smart lady told me "don't worry about it, if it's not slalom it'll be something else". You still have the drive and desire to push hard and excel so do another sport and do the best you can. I know it sounds like BS but for me it hit home. Staying fit is an important part of the formula but if we're that intense to push into short line and that capability goes away? There are lots of other places to put that intensity. It's more about the person than the sport.
He, your BIL, and I, will do ok. We'll find something and struggle a bit at first, but inside we'll push with the same intensity and drive to have fun, achieve a good level of performance and move on.
Talk to him about his personal qualities that got him this far and how they'll take him still yet further no matter what he chooses or does.
For me, It's the person and their heart, more than their leg, knee or shoulder.
Best of luck @rockdog