Can I just say, you sound like an amazing mum. It sounds like you are doing everything you can to help your son with transitions, while also processing your own emotions. Go you.
I would also keep advocating that visits being more child-led is in your son’s interests, although I appreciate this has not been met openly from his dad so far. You’re not suggesting a pattern for forever, but something in the short-term so your son becomes more comfortable being at his dad’s.
Are the video calls helping? I know it might feel hard, but i wonder if seeing you is adding to his distress. Of course, if he is asking to call you then it’s important to continue them.
Good luck, it’s so hard and you’re doing brilliantly.