Divorced Dad: Child doesn't like new girlfriend

How can I overcome my child's aversion to my girlfriend? I'm the father of a wonderful six-year-old boy. His mother divorced me when he was about three, and has since remarried. At present, she has primary custody and I have visitation rights. My son doesn't like to "share" me with anyone else and this has created a serious problem for me in my own social life. I even had to end an earlier relationship because he disliked the woman I was dating so intensely. Now I'm getting serious about another woman and am not exactly sure what I should do. Can you give me any advice?