Charlie,

I am in wonderment how friendly you can be with an ex who cheated on you and belittles you.

I am not sure what you are looking for.  Do you want to talk with your ex’s current gf on how to parent your son?  Is she parenting your son now?  When you ask how to handle “this situation,” what situation are you talking about?

Parenting your son is between you and his father aka your ex, not his current gf.

You are saying your ex has no interest in talking about parenting.  Is that all you are wanting to talk about?  Or are you adding his current gf in the mix of discussion?

Try to focus solely on co-parenting your son with your ex.  Make sure you come with a written down list of issues/items about co-parenting you want to agree on with your ex.  Better if you two arrange a set time and meeting place to talk about it with you sharing your list with him ahead of time.  Ask about what his concerns, etc. he wants to talk with you about your son for this meeting.

Mark