"Cosleeping"

[IrishDU]

Never actually used that term before, but I saw that thread and wanted to add my 2 cents worth.

I may not have fully grasped the nuances of everything you posted, but the take I get from it is this:  Your partner sometimes sleeps over, your son wants to crawl into the middle, and your partner objects.  
Sorry, but in my view, that's not right.
Whether or not your son should be allowed into your bed, is a separate issue, for you to decide.  But your partner needs to accept that you are a package deal.
Furthermore, if he expects to become part of your life, then that means becoming a part-time step-dad.  And honestly, there are a LOT more challenging things ahead, than "cosleeping."  Just waits till he hits puberty and turns into a little a-hole.

For context, a couple of years back I met a lady with a young teenage daughter.  I knew going in that for the past decade, they had only each other and were joined at the hip. 
I pretty quickly learned that she was precocious,  inappropriate, and without boundaries.  
So yeah, she still climbs into bed with us to have a conversation, or watch a movie.  And again, honestly, that's the least of the issues.  Having a teenage daughter again, is something I thought was behind me.