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Can he do the overhead, front rack archtypes?
If not that is where you start.
Mobilize those positions until he is able to replicate those archtypes.
It sounds like you’ve identified the problem areas. Start chipping away at them.
Does he started in an organized stable position?
Start with the spine.
If spine is not organized you don’t have access to the primary engines of the hip and shoulder.
Was he injured doing hspus?
His body has less tolerance for less than optimal positioning since the accident.
Has anyone looked at his push up technique and movement pattern?