Well the rule is:
Boss Mob: All models in this Formation must be deployed as a single unit. Models with the Independent Character special rule cannot choose to leave or join the unit.
Since the
SAG (5,5) result doesn't give you a choice I would imagine he leaves the unit and cannot then join it again afterwards. I would imagine as he is still an
IC he could join another unit potentially.
Really a question for
YMDC.