How can I teach a bitter man to be grateful for what he has?

Don't. People don't get this way because they are ungrateful or bad or whiny or stupid, and they certainly don't get this way because they haven't been berated enough about it. They get this way because there's some damage, physical and/or mental. You can't get inside his head and make an assessment, not that you appear to be trying. Things like this are so common, and the people so desperate, that a lot of money is paid to people many of whom can't do any good either. I have a friend who was in sales and cheerful all the time, and he was practically a motivational speaker. Now he's miserable and down. You know what happened? He fell off his bicycle. People aren't simple.

During my bitter phase of life, my bitterness was coming mainly from pain that was present in every day of the year. As I had to relinquish some pleasures in life to reduce this pain, the bitterness shifted to lack of those basic pleasures that I thought average person had. But at that point I was able to gather enough strength to let go of feeling deprived and start living. The entire transformation was rather unpleasant, time consuming, and complicated. Ask him if he is in pain, if he is, try to help him heal it. Real, chronic, physical pain cannot be easily wished away, otherwise we would not have House M.D. If the bitterness is from feeling deprived, the only thing that truly helped me was understanding Zen and seeking after the long forgotten existence in the current moment. A book that really inspires along these lines is The Practicing Mind. http://www.goodreads.com/book/show/335864.The_Practicing_Mind I hope that your friend finds a way out of the bitterness.