10 year old companion

You had suggested if my 10 year old hits his 7 year old sister again to preteach him he'll be my companion until he earns trust with her back. He did hit her again and now he's my companion. He's been my companion for 3 days now. How can I preteach him he's earned trust back and doesn't have to be my companion anymore and what do I do when he's power struggling and running from me during the times when he's out if instructional control and has no privileges, refusing to be my companion all day, day after day, and hitting her?