I just found out i have a 7 year old son, his mother doesn't want to let me see him, how can i see him

If you pay child support, you must have known you have a child. When you have a child and are not married, you need to file a Paternity Action (Petition to Establish Parental Relationship) to establish you as the father and address custody, visitation and support.  Child support is usually re-assessed based on the percentage of time spent with the child (visitation).