Sleeping with an ex is never a good idea. It brings back old feelings and never ends well.
If you've been chatting for a month...here's a crazy idea...why don't YOU ask HIM for his number. Girls today have to go after what they want, and if you really like this guy, ask him out.
If he thinks you're too forward for doing so, and doesn't want to meet with you, drop him and find someone else. You never know...most guys appreciate a girl making a first move once in a while.