Hello,
5 year olds love things to be the same, they do not generally welcome change. I don't doubt that the idea of having a stepmother (a major change, no matter how great she may be) is very upsetting. Though he gets along great with your boyfriend, he has also had a major adjustment to make recently in your house as well. It is up to all the adults to recognize how upsetting this can be for a child and to place the child's adjustment at a premium.
It is so hard on children to see one parent so infrequently as twice a month. Your son's relationship with his father is severely tested by this custody arrangement, so its no wonder that he feels protective of the little time remaining with his father. Not seeing a parent for a month at this age is an eternity, and I am sure your son wonders if he will ever see his father again. Giving your ex the benefit of the doubt, he may have been talking a lot about the wedding and the new wife to help your son feel like he is part of things, or to try to get him ready for the change. Fathers who do not get to see their children frequently are at higher risk for having the quality of the relationship suffer over time, particularly if they feel replaced by a stepfather-- I wonder what has happened here to change things.
So in terms of recommendations, I recommend you get the book Difficult Questions Kids Ask about Divorce as soon as you can. This very practical guide will help you talk your son through his fears. Expect that he will need lots of extra reassurance and support. Since your son can not do so, I recommend you get in touch with your ex and try to get him back in contact with your son as soon as you can. Don't blame and make accusations, just tell him what you have observed and ask for his help in making your son feel better. Finally, if your son continues to be very distressed, seek a psychotherapist for him with some expertise in these issues.
Best Wishes
Rebecca Resnik
Thank you so much for your help.