fear of doggie door

Question
I have a small dog of indeterminate breeding (probably part chihuahua,
though he is bigger than one).  I rescued him as a stray about 5 months ago,
and my vet guessed that he was about a year old then.  I had him neutered
shortly thereafter.  He was already house trained when I got him.  I had a
doggie door installed, but I cannot get him to use it.  When I tape the plastic
flap up, he goes in and out freely, but the minute I put that flap down, he will
have nothing to do with it.  I have tried to induce him with treats, a ball (he
loves to fetch), a leash to indicate we are going on a walk, but nothing works.  
Based on advice from another site, I tried gently nudging him through the
flap a few times, and then left him outside and tried to get him to come in.  
All he did was sit forlornly, literally trembling with fear.  Is there anything else
I can do?  He is just terrified of that plastic flap.  Thanks for any advice you
may have.

Answer
I often share this article with people who ask me about dog doors and fearful dogs:
http://www.collargirl.com/dog-door.html
I hope it helps you, too.
Use really scrumptious treats if you need to coax the dog - things like liverwurst, tripe, Plato salmon treats...etc.