What are some reasons why a person would get acne on only one side of the face?

Recently my husband has been getting a lot of acne on his left side of the face. They're hard bumps, sometimes raised but nothing in it, sometimes white heads but it's concentrated on the left side. We used a lot of different acne medications but haven't found anything to work. Last time he popped some of them and it just bled (a lot). Once the blood came out the bump went down. (Sorry TMI) I really want him to have clear skin and it makes me sad that he's suffering through this. Any advice? Thanks.