German sheperd barking at his own reflection

Question
Well the 9month old german sheperd we have got was ok, until a couple of days ago, he started barking at his own reflection.

Its a pain, coz he does it in windows and doors, and its not like we havent tried explaining its ok.

So any suggestions why the silly so and so has started this. Our Labrador who is older is fine and has never done this.  

Answer
Hi Jenn;
I think he knows it is HIS reflection.
Stand there with him so he can see your reflection beside him, if he doesn't know it is him, that should teach him after awhile.
We had a cat once that would see her reflection in something, and she would stand on her hind feet, put her ears back and rapidly paw the mirror, bookshelf ot whatever she saw her reflection in.
Don't EXPLAIN it is ok, she him you in the reflection with him, teach him what it is, then correct him wioth a sharp "NO! don't bark there!"
Charlotte