Because they are inherently selfish people. In order to do what he wants your WS has to believe that the kids will be just fine, heck everyone will be just fine.
I am sorry you and your son are suffering. I remember sobbing and sobbing the first times my kids were forced away from me due to WS.
What can you do to build the confidence and strength in your little boy, so he can be OK away from you for those days? The reality is you have to work..
Can if be a good thing he develops the ability to separate and then reunite with you with ease? What are things he can take to help relax him? What are things he can do to entertain himself?
I remember when I had my second child, the first one hated it (at 20 months old) when I had to nurse the new baby as she wanted all my attention.
So, I got a backpack and filled it with novel things and that backpack only came out when baby was being nursed. This helped her make the separation.