When we rescued our pup Dexter, we had set feeding times and only allow his bowl to be on the ground for ten-15 mins then put it back up (although he ate every last little crumb before the 10 mins was up) This helped alot with predicting potty training for us.You may want to try that instead of leaving the bowl down all the time.
Also when it is feeding time you may want to try having a handful of tiny tid bits of "super treats" Cooked chicken, hot dogs, when your pup is eating you can start to bring your hand to the bowl with a tid bit of hot dog/chicken and hand feed over the bowl, when he stops growling i would treat, praise,pet him (sides only), be careful at first if he nips at you, i would yell Outch/yelp like a puppy ignore him for ten seconds then start over, keep repeating this over and over until he starts accepting your hand coming near the bowl as a good thing your pup with start to associate a human hand coming near the bowl as a great reward instead of a threat that his food might be taken away. After some time when he starts to trust a hand going near his bowl, during feeding time i would practice taking the bowl away with food in it and giving it right back. Good luck