Tell the builder that (a picture of what you saw would have been nice too) what he installed is a "service access point", and that what you are referring to is a "trap and a vent", and that with a "trap and a vent, the vent is after the trap".
Then thank him for installing a service access point, tell him that most builder's do not do that (actually, I am finding that many are, but that way, you can end with him being able to puff his chest up a little
).
