how do i go about getting partial custody of my son?

Get a lawyer.  If you can't afford one, go to Legal Aid and see if you qualify.  But also prepare to be a parent.  I don't know how old your son is, but he will need a dad who keeps him out of the middle of whatever is going on between the adults, who is going to be consistent and is going to think of his needs first.   If the threats are that you won't get time with your child, then filing a custody action in court will help set some ground rules about your time with your son.  If you and his mother can agree once there is a court action, you two may be able to work together in some ways to share the responsibility and the joy and give him two parents, even if they don't live together. Whatever the drama is between you and his mother, you have to be prepared to step up.  Find a parenting class locally and take it.  Make sure you are contributing to supporting your child.  The law recognizes that whenever possible, children do better with two parents in their lives.  If you are doing what you need to do, and are focused on keeping him out of the middle, you will be a good father and every kid needs a good father.