Dotsie,
We have the same problem and he's not a teen anymore. Now I just tell him look at the calendar.
When he doesn't and asks me to do something, if I already have plans I don't go. If I am available I go.
Its real hard to turn them down too but it does get better once they learn you aren't available at their whims. Chatty I like the way you do it too.
Maggie