I want to tell my man that I want to step up to the mommy plate to his son like he has mine, but I don't know how to tell him. I don't want to say the wrong thing and I've never been in this situation before. What should I do?

Well I would sit him down and let him know tht first off you appreciate all he has done for your son and then that if it is ok with him you would like to do the same for his and grow a bond bc tht is really important in any relation ship hope this helps ^_^