Melena,
It sounds to me like he only bites when he is intimidated. Is that what you are saying? If so, I personally feel that maybe you could help him. It just takes a very long time and a whole lot of patience.
I had a biter, my Maxwell, for 11 years and he bit me often in the beginning and much less in his final years. Yes...I am saying that he did bite me still after all of that time; but I grew to love him probably more (and I hate to say that) than any of my babies. He was a very special guy who really only knew to use his teeth to defend himself. Heaven only knows what happened to him prior to him walking in front of my car many years ago.
I now have another biter who is very similar, but thankfully has not bitten me quite as many times. Maybe I am just getting better at avoiding it. As for yelling and spraying water....I found those things to be quite useless. It only seemed to reinforce the fear they had that something would happen if they protected themselves. With Matty, my new one, when I see his head go down, I simply walk away from him. It is normally triggered by something such as an accident he has had in the house (I am sure he got in huge trouble for being a dog)
... or if he sees something new. I took the ironing board out the other night and he was in the corner looking frightened. I tried to call him over and he would not budge, so I ignored him. Eventually, he walked over and sniffed it. I then praised him and let him know what a good boy he was. I was advised by a trainer to let Matty know that I am the alpha....well that did NOT work...at least not in the way that they insist. Matty knows and listens to me...he is just frightened. I plan to continue only positive reinforcement with him for some time. I believe he will change just like Maxwell did...once they know they are no longer going to be hurt, they settle. As I said...Maxwell did not totally lose the fear...but he got to be such a wonderful pet and my friends all loved him. Everyone knew what things bothered him and we avoided putting him in those situations.
All in all it takes a TON of patience and it is not for everyone. I can hear in your message that you really care, so I shared my story so that you can take another look. In the end you might still believe it is too much...or maybe you can try again...and let him go at his pace. It could be much much longer than a few months.
Best wishes in whatever you decide to do. I think it is wonderful that you tried. It is so sad that people ruin these poor babies.