How do I go about this? My son's mother doesn't have any court orders stating I cannot see my son but isn't allowing me to see him can she do that?

I am 23 years old, my son was born in September 2013. I am in the process of joining the United States Army to build a stable life for my son. I was told by my sgt in order to join I would have to give up full custody of my son to his mother. She is making things very difficult because we don't get along personally. She isn't letting me see my son she sends texts to my side of the family threatening to not allow them to be a part of his life. I moved away from her and my son in October right after he was born to train to enlist in the military and returned in December. When I returned she started telling me that he's HER son and that I have no say cause I was gone for 2 months and telling me to take her to court and that I wouldn't dare. Please help. I feel like she shouldn't be able to get away with this.