New to military life. Q about custody

My SO finished up 4 years with the USMC and we met when he was finishing his BA in college, he plans to join AF as an officer come this month. I have a son from a previous marriage and I'm super worried about not getting custody to take him with me. has anyone been in this type of senerio? I have one daughter who is 1, and a baby on the way with my husband, I don't want my oldest son to be ripped from his siblings. his dad is in his life, not a great person, but it seems like the courts favor dad's.