My Ex-Husband has suddenly changed

I am not sure how to handle my situation. I have been separated for 12 years, divorced almost 1 year and we have a 12 year old son. I initiated the separation but we always got along and co-parented our son 50/50 without any issues whatsoever. Throughout the years we have spoken decently to eachother, I have given him clothing, kitchen and home articles. I have even given him food for his parties and collected boxes for when he moved. He works for me twice a month and I pay him. A few months ago (maybe early Spring) my ex started bringing my son home late without any notice or phone call. When I would question him about it he screamed and had a tantrum. Since then, he has been barking orders at me, he has smacked the back of his hand into the palm of his other hand up in my face ( a gesture I take to mean ‘I want to hit you’) and he even forced me to take my son to Cadets on one of my custodial nights. He yelled ‘I don’t want my son sitting here on Tuesday nights doing nothing’. That was in July. Last week he forced my son to quit cadets as he feels it will only interfer with his schooling. I told him that it isn’t an issue and it’s on my night. He resonded with ‘I am going to have to use my parental authority on this one and say no more Cadets’. It’s like there’s two people. Today he yelled at me infront of our son and upset him to the point of tears. My son always calls me upon leaving school but today he didn’t. I had called his grandma to have him call when he arrived there and she said fine and added that he is probably late and forgot to call because he’s with his dad. I proceeded to call my ex and he told me not to be calling my son as it’s not my problem if he’s late and hasn’t called as it’s not my night. I told him that it was something our son never failed to do so I was worried and that’s why I called. As I waited for my son to get to his grandma’s I received a call from the school. His science teacher told me that his assignment was due two and a half weeks ago and that she would have to fail him if he didn’t hand it in by Monday. As the weekend was kicking in I decided to meet my son at his Grandma’s to discuss this with him and take him to school to pick up his work if he did not have it. This was the onset of a war. My ex-husband told me to stop speaking to my son, to say good-bye and get out. he pointed to the door. I told him that I could take him to the school to pick up the work before the doors closed as I have a car and he doesn’t and the school is half hour away walking. He told me it was none of my business that he would handle it and to get out. I knew he had to be at work and that he could not get him to the school to pick up his assignment. He yelled some more, said “Fine, take him and be back in ten minutes (impossible due to distance) and never come here again”. I told him that I was there because of the teacher’s call and I didn’t want him to fail. He replied , “I am pissed off and your problems are about to get bigger”. I asked him what the hell he meant by that. “He said, “You’re going to see”. Can anybody tell me what to do? I have no idea who this person is? He’s gone nuts. I fear that he’s going to do something. We have a trip planned for next wee (my son and I) and I am scared he’s going to sabotage it.