Hi. There are definitely some things you can try to help break this habit.
1 - Diet is really important. He may not accept it at first, especially if he's used to a crappy diet from his old owner (which it sounds like he may have been) but you have to just give him healthy food everyday and eventually he should start eating it. Try eating it in front of him and making a big deal about how good it is. Sounds silly but it often works. Anyway, he'll (or she?) will need a good quality pellet, either Harrisons or Roudybush, something with no dyes. Also, a variety of fresh foods daily, especially orange veggies like carrots and sweet potato and even pumpkin. Also, greens such as kale and any other veggies you may have excluding avocado which you should never give to a parrot. Mine love peas and broccoli and corn etc. Fruit is good too but veggies are better. There is an excellent recipe I use every day that is really healthy and easy to make and all of my birds really love it. It's the first thing they go for in the dish though it took them some time to get used to it. It's called glop and here is a link to the recipe -
http://www.companionparrot.com/glop.htm Be sure to add the essential fatty acid! I use Flax Seed Oil as I've read that it has helped with plucking in some birds. This recipe may be the easiest way to get him to eat good stuff. I actually make enough for several days and keep it in the fridge.
2. Make sure he has toys and stuff to keep him occupied. Greys are really intelligent and they sometimes pluck out of boredom and frustration. It may take some time to find out what he likes to play with and he may not even appear interested in playing. Some birds have to be taught to play. If he won't play with anything try to play with it in front of him and really act like you're having a good time. Sometimes just making it clear that they can't have the item is enough to peak their interest. Also, quality time with you and out of the cage will help with boredom.
3. Lighting. Many birds have stopped plucking with the simple addition of an avian UV light, especially if they have been kept in an area with little or no sunlight.
4. Some birds have had success with anti anxiety drugs which is something your avian vet can help you with.
5. A bubble collar, another item you can get from your vet. It simply clips on around the neck so the bird can't reach itself to pluck. It's extreme and the parrots hate them and I'd really only recommend them for extreme cases where mutilation is occurring simply because they seem so torturous for the bird. The bubble collar is better than the Elizabethan collar because it is much easier for the bird to move around and eat etc with the bubble collar. A homemade foam collar can work as well if the bird will leave it alone. You can get foam pipe stuff at Home Depot, it comes in different sizes, and cut it into pieces about an inch or so depending on the size of the bird and just slip it on. If the bird won't leave it alone you can wrap it with vet tape to help keep it in place but be aware that sometimes feathers can stick to it and get pulled out when you take it off.
5. You can also try a humidifier in the room he's in and (even it he hates it) a bath at least once a week, preferably more. Some people mix pure aloe vera juice (not gel) with water and spritz their birds and claim it helps. Sometimes they pluck because of dry, itchy skin.
So, those are some things you can try. Some birds will stop plucking with a few lifestyle changes, other will pluck for their entire lives. I wish you and your bird luck. Dealing with a plucker is no fun. I have one that plucks her tail feathers out and nothing has worked to get her to stop. From the picture, the plucking on your Grey doesn't seem too extreme so there may be some hope for him!