A.J.
I too went through this with my kiddos. We had a very messy divorce and once my ex was allowed visitation, my oldest son who was 6 at the time would scream and cry and fight us just like you are describing. The other kids were just fine with it and it broke his dad's heart. He also had trouble committing to call and talk to him in between visits so that made things worse. So, I feel ya!
Now, things are great and it seems like a faded memory even though it wasn't too long ago. My son is 8 now and talks regularly about how he loves his dads (which while he still calls my bf "mr matt" he says he does everything a dad does)
I just stayed positive, and encouraging at home and during the drop offs. I did not leave him with dad if he worked himself into hysterics but, we would sit on the porch or have lunch in a restaurant and try to make polite small talk and ignore the tantrum rather than doing a "hit and run" style drop off car to car. This made him much calmer and removed a ton of the stress. Right now his dad is a stranger to him, watching you interact in a cordial polite manner will remove some of the feelings that you may be selling him to gypsies.
I also agree with comments below that you should be receiving on going support from dad. It is called a temporary order of support, and as long as you have an "active" divorce case pending, which means you have at least filed the initial papers, you can apply for it.
There are 3 ways you can do this, and please message me if you want help with this-
!) I found the temporary support forms online for my state and wrote them up myself and brought them to the courthouse on the day the judge saw walk-ins and got him to approve it. I then faxed copies of the signed order to my ex's employer with a request to draft his account and my mailing address to send the check.
2) You can talk to your lawyer and they will do this for you, but they'll charge you and it can take weeks or months to do what you can on a Wednesday morning.
3) You can call the Office of the Attorney General for your area and they will send you a form to complete with all your ex's info. Within 6 weeks they can start docking his employer but then it goes through the child support division and you might not see it for another 120 days.