There is evidence out there that a LOT of shoulder surgeries end up with a STRONGER joint than pre-injury. How his re-hab went is the key but sounds like he has been doing all the right things .... now ... the MENTAL part you speak of ..... THAT IS key! Will he trust the shoulder? Will he forget the trauma of the injury? Will he be more in tune with being BETTER than he was last season ... or .... be thinking about just being out there?
The mental "rehab" of these injuries are as big, if not bigger factor in the success or failure of the repair. As mentioned in the Gurley threads about knee injuries ... most further injuries involve other joints because the player is still trying to "overcome" the original injury.
If you feel good. If all tests of strength and atrophy measurements are a go ... you have to "forget" and just go. Can't be scared or thinking about what has been already fixed.