Can something be done

4 yrs ago, I left my husband and took my son, started divorce proceeding, and moved in with the man I was going to marry. My husband hired detectives and the worse thing in court that was said by them was the guy I was with, my son and I, "looked like a happy family". It was the grounds the judge gave sole custody of my son to my now ex-husband. About 2 months after the divorce, my ex took my son and went to another state, giving me a one hour notice. I was devastated! He lived with another woman, with my son, and eventually married her. After one year, they are getting divorced and he is waiting until it is final in Oct of this year, to marry another woman. The thing that bothers me is him living with these women with my son, who is now 6, and claiming it is different. He is married to this one woman and living with this other and with sole custody he thinks he is invinceable. My son is old enought now to know they are sleeping together and that bothers me. Isn't there ANYTHING I can do!!! Please help!