I'm sorry, but a 5yr relationship went down the swanny a couple of YEARS ago? and he's still in bits?
All the while he is mucking about with you, sleeping with you, cozying up to you, expecting you somehow to be there to look after him when he's ill, but has made it crystal clear in the past, and now is actively freaking out at the very notion of being in a relationship with you?
You are potentially on a very big hiding to nothing here.
He doesn't want you ENOUGH to have a proper relationship with you. Oh he'll get his needs met alright, but no commitment.
Regardless of what he says about you and he remaining friends, no sex, I promise you that in a week or so, he'll be sniffing around for it, and you being needy and vulnerable you'll let him.
BASICALLY, as hard as it is, you need to understand that calculatingly or not, this guy is USING you. You in turn are ENABLING him.
My suggestion is that you cut ALL contact, as your presence in his life will only form to confuse him, and his presence in yours will only serve to tempt you.
IF he gets his head straight and decides to come back to you, you will be in a much stronger position to decide whether you have to settle for this, or if, as I suspect, you might be better than this.
This is a goddawful mess you are in there, and allowing him to stay around you will drag you down with it.
You are apparently the ultimate Friend With Benefits. Stop this now, for the sake of your sanity and for the sake of your son.