OP YANBU but not sure there is much you can do. If your youngest doesn't want to go, don't make him. Email his DF:
Dear ex
DS6 has expressed genuine unhappiness at your new living arrangements. His main concern is the sleeping arrangements, in that he is expected to share a pull out camp bed with his older brother. He feels acutely that you don't value him and that he cannot sleep properly whilst cramped up to his brother like this. As you acknowledged previously, sharing a double sofa bed did not work out well for the boys so I'm sure you can see why sharing a small camp bed is not going to work out either.
This is so upsetting for DS6 that he no longer wishes to come and stay. Please let me know when you have sorted out sleeping arrangements for the boys and what they are so that I can reassure DS6 and he can continue his visits to see you.
On a different note, you will remember, I'm sure, how important it was to you to know who my DP was and to meet him when he moved in with us. I respect you as the boys' father and did everything I could to show that, by facilitating you two meeting and giving reassurance to you about who our sons would be living with. Please also remember that, just as you are their father, I am their mother and have just the same concerns as you about who they may be spending time with. It would have been good to know that you were moving in with a new GF and at least where the boys would be on a regular basis during your contact time. I will continue to act respectfully towards you as their other parent and would ask that you do the same for me, so that we can co-parent effectively to the best of our ability to benefit our boys. I have no intention of interferring with your contact time in any way and just ask that you keep me informed about where they are regularly based and who they are regularly in contact with.
Please let me know as soon as a bed for DS6 is arranged, so that contact visits can continue uninterrupted.
Thanks
Mar