
I used to have a GSD that could be picky with her food, I think it depended on her hormones, she was very temperemental at times :P
But it is frustrating having dogs that are picky. My older boy has days when he'll leave his food and other days when he'll scoff everything in sight :rolleyes:
I feed mine on raw and know that if I went back to feeding all in one, they would turn their noses up, but that is just my 2.
I think you need to experiment a bit with different foods, not saying spoil him ;) but see which food gets him most interested. I have heard very good things about Naturediet (haven't tried it myself, haven't had the need to) especially if you heat it up. Also, if he gets a bit constipated again, which is common in fussy dogs, try giving him some liver or other offal, very good for loosening up the rear end :P If you can get him to eat it of course..........
Good luck :)