A 7 year olds perspective is not the real world.
My 2 DSS's (1 being 7) think they should be allowed red bull, alcohol, unlimited sweets and chocolate, chicken nuggets and chips for every meal, unlimited screen time, not to have to get dressed, not go to school, not to have to brush their teeth, not to have to shower or bath, not have to follow rules and to only think of themselves etc. Whilst we take their feelings in to account, none of these things are realistic or aid in raising children into functioning adults.
Add on for OP's DS a mother that is enabling their negative feelings because of her own negative feelings.
If the bed is always made up/ out when DS is present, he will know no difference. Unless 1 is a camping bed with newspaper as a duvet and the other a 4 post with a luxurious duvet, which of course would be unfair. If this was a nuclear family who went on to have more children, the only difference is that they would have a bed in situ permanently. This takes up a lot of valuable space, space that 99% of us don't have to lose. So why should DS have a bed out constantly taking up space that is needed 7 days a week when he only needs that space 1 day a week?
DS now gets actual quality time with mum at weekends, which he didn't have before. But all the anti dad posters are failing to see that when mentioning the reduced contact.
Perhaps mum and dad should have been adult enough to say 'full weekends don't seem to be working, let's do an evening/ night in the week instead'.