My Grandson Who Lives With Me Is 24 Years Old

My grandson who lives with me is 24 years old and has severe emotional problems. He can be fine one minute and have a abrupt mood swing the next. He thinks his life is not worth living and can see no point for being on this earth. He thinks everybody around him treats him like dirt. Whenever he starts with one of his speaches about himselt, he says the same words over and over from day to day. He alsaw sways back and forth from side to side continually. I have begged him to let me help get him some help, but he says it won;t do any good. I don't know what to do. Do you have any suggestions?