I've never really dealt with this, but it seems like going completely dark is really only for situations where the WAS is showing NO INTEREST at all.
Seems like one possibility if he's initiating contact is to accept some and "be busy" on some others. Maybe try to keep the together times short and extremely successful. Maybe show him how great things could be. But make him initiate all the contact now, and don't be available for all of it?
I'm thinking out loud here, but maybe the message you want to send (nonverbally) is "I DO want this to work and things can be great, but I've also got to get on with my "new" life in case that doesn't work out, and while that's not what I want, I'm prepared to accept it and be successful at it."
You keep using that word. I do not think it means what you think it means. -- Inigo Montoya, 'The Princess Bride'