Most Helpful Guy
You can't. Not really. Friends with benefits requires a solid friendship and no emotional ties. You are an ex. You will have emotional ties coming out the yin-yang. It will not be possible for it to remain on a friendship level, somebody is going to get hurt, and it is probable that any kind of friendship will be destroyed even more.
Much better to look for a friends with benefits that you haven't had a relationship before with if you want it to actually work for any extended period of time.
Most Helpful Girl
Bring it up and ask him would he like to be your friends with benefits