Is there anything that I can do to try to have visitation rights?

I was married young and my ex had a son that was 10 months old when I came into the picture. The birth mother wanted nothing to do with him and signed away all of her rights and has not had contact with him in 4 years, nor does she want anything to do with him now. The son never considered anyone mommy but me. I was there and raised him and love him just like my own. Long story short, my husband and I did not work out. He refused to let me see him and two weeks after we broke up her was with someone else, 3 months into their relationship she was pregnant. Now the son has been moved away from everyone he knows with 2 other kids, the girlfriend has 2 daughters, and a little brother on the way. We are still married and I plan on starting the divorce soon. Thank you for any advice/help.