Dear J.,
I am so sorry that your son is going through this. I am a stpemom and my stepdaughter lives with us on a full time basis with visitation with her mother. So throughout my experience with this I can try to tell you what has worked for us. For a while my stepdaughter lived with her mother and everytime she would visit us it was a fight to get her to go back to her moms, but my husband had to make hard decisions and she had to go. Eventually, we have worked out something where she now lives with us and has done so for the past 3 1/2 years. Although our situations are different, it still deals with divorced parents and the stress it puts on the children involved.
1. I would put your son back in therapy if he is not already there, it is worth it believe me, it may seem like it is not doing anything right now, but it will help!
When your son cries you need to be the strong parent and as hard as it may seem you have to be the one to tell him he has to go to his father's. And I am sure this is what you are doing it sounds like, but if everytime you allow him to run into your arms and comfort him after and before visits, alowing him to cling to you, then he is feeding off of you that something is wrong with the visitations and father with his new family. You need to stay upbeat give him a smile and tell him he is going to have a wonderful time give him a quick hug and kiss and send him on his way. When he comes back greet him with a smile, reinforce that he had a wonderful time ask him about his time, what they did. After visits with my stepdaughter to her mother's we would make a big deal about it and let her know that we knew she had a great time and ask her all about her visitation and be really positvie about it. And she would start to realize that maybe it wasn't so bad after all. I know your son's 4, but I have a 4 yr. old as well and would still talk to him in the same manner.
2. Stop allowing him to cling to you, that is the worst thing that you could do and I know it goes beyond all your instincts as a mom and if you have to get in your car and cry it out later then that is okay, just don't let your son do what he is doing. It is only going to hurt him in the future and you are reinforcing the behavior.
3. When he asks you if he gets to stay with you. Show the same amount of excitement as dad's visits as you would if he was going to be staying with you.
If you are showing any sort of disapointment at him going to father's or even a slight change of tone different than what you would express if he was going to stay with you. He will pick up on that and right away think that something is not right with going to dads house. Again, stay upbeat, act happy that he is going to dads, reinforce what fun he will have, and let him know you will see him when he gets home.
I know this is very hard for you and I have watched my husband deal with this and the stress that it has put my stepdaughter through. You can only control your behavior and a child having both parents is the best thing that you can do for him, but your son alienating his father and his new family, will only hurt him. You have to be the strongest and not allow it.
Things take time, kids need to adjust, there is no magical answer to the problem, and this is a huge stress on your son not having his mom and dad together, and now dad has a new family and your son is not getting all of dads attention. It is very hard, and sometimes we have to do things we don't want to do.
Keep your son in therapy, it will be worth it.
And I am so sorry that you and your son are having to go through this.
P.S. After I read through some of the responses, about people saying something is wrong at your ex-husband's house, is not always the case, and tread carefully if you accept that advice because you would hate to destroy you son's relationship with is father for no reason other than paranoia people giving out advice. Make sure you have all the facts first before you act. I'm not saying don't acceot that advice, just make sure you know before you act on anything. As for sending your son without notice, I would definately not do that either, that would only traumatize him. The fact is that your son is now just becoming aware that his mom and dad are not together. The are going to be reactions to that. My husband and his ex were divorced when their daughter was 2. We only started experiencing problems and adverse affects to that when she was about 6 and mom remarried and dad remarried and she was confused and didn't understand why her mom and dad could not be together. And her new brothers had a mom and a dad and she didn't.
Best wishes to you. I hope this will help.