I'm with VHG on this - you have to reassure your girlfriend that if you were going to be with your ex-wife, you would still be there, and that it is your son that links the two of you together, not your previous relationship.
Equally you have to make it clear, without appearing to issuing any kind of ultimatum, that your relationship with your son is part of who you are, and your ex-wife forms a vital part of that, so it is going to continue.
She will then realise that she has a choice to make.