Well, based on what you told me, he can't prevent you from using the alley road which has access to your side of the property. Unless he can present a document or agreement between yourselves as co-owners of the property that he has the sole and exclusive use of such alley road. Further, I don't think there's any basis for his demand or claim of additional compensation.

You can consult with a lawyer who will write this person a letter to desist from any act preventing you from using the alley road. HTH!