Weeeell, I don't think spirits would be wandering, even given how every Angel and Demon that can is fighting on the Human world. The Void and the recycle of life(except for humans) remained untouched(Except where Mary is, but that will be explained in her CS)

I do see a way he could keep his magic and not have anything really changed. The void for Angels and Demons isn't a long stay. They're there, and then they're gone, ready to be reborn. Humans, on the other hand, aren't reborn. His magic could be centralized around pulling souls from the Void. Obviously, these would be primarily humans, but if he's fast enough, he could conceivable snatch an Angel or Demon Soul.

That's my two cents at least.