Start making dinner for just you and your son, or go out to dinner alone with him on days she doesn't say she's going out. When she asks questions, say something like, well since you usually have plans, I decided not to waste food or time by making dinner for 3.
I thought about doing that too. I think I'll just expect to cook for me and my son only from now on. If she wants dinner, she'll have to call me to let me know when she's coming home, else she can reheat the leftovers herself.
