Boyfriend is cosigned on my auto loan. I am about to leave him. Can he take my car from me?

Anyone who owns the car, i.e. on the title, is allowed peaceable possession of the vehicle. He can take it, and so can you. Neither can use force against the other. You can take the car if you leave, but he can as well, if it is done without force. You may wish to leave and take the car yourself. "Primary" doesn't have a lot of meaning on a loan or a title. Both are equally liable and equally entitled to possession. You should consult counsel as to who to resolve the issue between the two of you.