Another question: what kind of deal does he have with this spirit? Sponsored debt might also be an option.
If you combine it with Involuntary human form, in times when the spirit says "No Way!", He can choose to take a point of sponsored debt.
This way he gets access to his powers even when the spirit may disagree, but then the spirit has a free compel to use against him to further its goals!
The original deal (this is all part of the backstory) was the spirit was inprisoned originally and broke a deal with the character (a child at this point) that she would teach him magic and in exchange he would release her once he reached an appropriate level of magical know-how.That all became moot point though when she became permanently bonded to his soul. Now its just sort of a fact of life for both of them. I think the idea is that the connection to the spirit weakens when he strays too far from her nature. Sort of similar (Cold Days spoilers) to Harry's mantle of Winter Knight in Cold Days
. The more he acts against it, the bigger affect it has. Its also probably worth mentioning that they aren't enemies. They may not be friends exactly but they have been together a long time and are partners.