What do I have to do to get my son back from his father that has threatened to take him from me and move to another city or state and he has him now

Me and my ex have been separated almost a year now we got into an argument in December 2017 from there things have gotten worse but I was still letting his mother get my son even after he threatened me via text message I let his mother get my son Friday before last for a week when I called to get my son she stated that my ex had came to get my son do when I called my ex he was very hostile and cursing about not bringing my son so his mother said she'll have him bring me my son yesterday so I waited around for them to come they never showed up So today when I went to get my son his mother called him with lies saying I disrespected her etc All I asked her to do was call her son because I felt like they were trying to keep my baby from me she the one who went off on me for no reason because I said I left my son under your care and ask you not to let him leave so my calls threatening saying he isn't bringing my son back he's taking him if I get the police involved he's going to come to my