I assume that by hostile you mean red..
Is it possible that he got charmed?
Did you attack him?
Did ANY aoe spell casted by you affect him?
Did you kill his family in government district? (im not sure if its possible if he aint in your party, and even if it is, it might not affect him in vanilla game, but..!)
Are you a chaotic evil child killer with 1 reputation point and personas like : Valen in your party?
Have you wiped out Order of the radiant heart? (again im not sure if it affects him in vanilla game)
Have you spawned a neutral (innocent) NPC right next to him, and killed in cold blood?
If none of the above is true, its a bugg.
If he is blue (neutral) try talking to him. Try waiting a bit till he is out of combat script. If it fails, reload a previous save.
If you want to simply join him to party and dont care about the first dialogue too much, CTRL + Q him (debug mode needs to be enabled)
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