You've already allowed provision where a soul and spirit can each exist without the other (in the cases of resurrected minions and zombies), so personally I don't think it would be too big of a stretch for his soul to temporarily part with his spirit. That would also mean that he would still have access to the raven's spirit, allowing him to use its instincts in order to fly (because let's face it, learning how to fly without have a bird's instincts would really suck). It would also explain why there would be a range or time limit on the possession, since having he spirit separate from be soul for too long would be detrimental to both.