Hi H.. This is just my opinion so feel free to use it or not.
It seems that the mom is keeping your step-son's schedule secure and simple by not switching weekends. I am sorry that she is not more lenient about it but she doesn't have T. by law or otherwise.
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I would recommend this, arranging a time to call. Make sure the mom has a time scheduled so she can be sure the boy is at the phone. Have your husband schedule one hour away from everyone and everything. Ask him to have different topics in mind to discuss so there is no dead air time and plan on listening time too. Then call. Do not promise and then not call.
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Do this also just in case the phone call falls through on the son;s end.
Purchase a card and have your husband write a note to his son. Miss you, Love you, Can't wait to see you. Then if he would, write something important like what he felt the first time he saw his son, or how cool it was when he taught him something they both enjoyed. Then ask his son to put it under his pillow and when he is missing his dad or he is sad, he can pull it out and read it and know he is loved.
Many dad's work away from home and have a good relationship with their kids. It's all about communication and making the child feel that they are loved and are important in their parent's world.
Is is so kind of you to care about their relationship. I would be lonely for my child too if the time was so long.
God bless, C.