hmmm. Visitors visa switch to Leave to remain? Im not sure but I dont think you can do this (I hope Im wrong) even though you've got a child and another one on the way it doesn't mean he have good chance of staying with you unless he gets his marriage annuled first and both of you get married. Hope you dont get upset on my comment and im sure some other members soon will say their knowledge.
I just wish you a goodluck for the future at the moment why not visit the Home Office and British Embassy's website?