My child came home crying and won't tell me what happened

Recently my ex got part of custody of our 14 year old son. He gets every weekend. He hasn't seen our son in 2 years. He went there yesterday and was supposed to stay until Sunday. I see a car in our parking lot at like 3 in the morning and its my ex's. I see my child run out the car. I go to meet him at the door and he is in tears and pushes past me. My ex drives away fast and my son runs into his room and locks his door. I knock asking what happened and he said to leave him alone. I call my ex and he told me to fuck off and "That fucker can stay your problem". My son won't unlock his door and keeps telling me to go away and refuses to talk to me about what happened at dads.