Yes, Your brother can be helped. It is nice of you all, his cousins to be supporting him in the way you are. I am sure you also might have tried making him see possible reasons for his loneliness. At this point he either lacks the complete understanding of how he is perceived by others or does not know the ways to overcome the situation because his behavior seems to be coming from his insecurities and lack of self assurance. There can be 2 ways in which you can help him further. Firstly does not pay attention & ignore him when he is being boring however point out & appreciate little things he does right. Eventually he'll do more of what's appreciated & less of all that which does not get him any attention. Second way is to help him see the importance of listening. Whom do we usually like in our life..?? Those who like us right..!! and how do you show that you like someone..?? By listening to them. When you listen to anyone keenly, they already find you likable. Try all these for a month and check if there is any improvement. If not, do consult a psychologist/counselor for further assistance.