Well, my thoughts are likely not going to totally satisfy either of you. Maybe I will come accross as a little narrow in saying this, but some might call this sleeping arrangement at least a little inappropriate. These girls probably do not need to be in the same room as your son. That is not making any accusations and nothing really wrong with it per sae. It is just may not be the best situation.
That said, I tend to agree with you. This is your son's home and his room. His space needs to be respected. On the other hand though, for a few days a month it is also the girls' room. They need some space to call their own as well. As an alternative, you might be able to place a hide-a-bed or sleeper sofa in the livi g area.. When they visit they get claim on that area of that section of the house. I know this is not ideal, but it does seem to be the most fair and appropriate arrangement in your current situation.
Not to slight your husband's predicament, because it certainly is real, but another option is to purchase or rent another living space to accommodate his daughter's visit. If that is not reasonable, you are going to have to do the best you can with what you have.
I understand your husband is trying his best to be accommodating to his children in the short time he has them. He wants them to feel like in the time they are there, they are home. Though I do believe moving your son out of his room is not the solution, I do feel that every other effort should be made to give these girls a sense that this is home. This arrangement is not their choice and they should not be punished for it. I am not sure what this means, but I suggest you and you husband pull your creativity together and find a solution that is suitable for all the children.
I hope that was helpful. If you have questions or wish to bounce ideas off me please feel free to. I really do want to be helpful to you and your family. Thank you!