Firstly your main aim seems to be having the father around your child. So with this in mind.... Tell Omari that he will have a child soon and needs to sort out this Mark issue as it is stressful to you. Women that are pregnant want certain things to be "set in stone" such as who is going to live where and what the arrangement is going to be. He needs to step up to the plate and realize his life no longer entirely revolves around himself if he wants to be a decent dad. Decent dads make decisions that benefit their children.
Just make sure you frame it around him taking action as the man and it should give him enough motivation to do what is needed.