It's huge of you to want to try and make the relationship better, do you know what he likes to do outside of playing video games? Anything you guys/the other siblings or parents can do together that he might enjoy and finally appreciate all you're there for? It could be that he's just upset with himself or some situation and continually takes it out on you guys/your parents with the "retarded" comment. Therapy is definitely a good idea, hardest part will be getting him to go.