mmm, not so. i went save the mages route, even if i wasn't intending to. he left me at the initial confrontation with the big kaboom scene, and a bit later on i told him 'hey, guess what, i want you back over here cause you'd kick my butt all over the place'(not exact option, or even paraphrase). one line of diologue and he's like 'yup, sorry. i'll be with them again. apologies for the confusion.'
note: did require full rival
so he would turn, but if you have full rival, choosing the right option will bring him back to your side.
Doesn't need full. I managed the same at about halfway there.