First off, Welcome to WrongPlanet! Your son will enjoy it here and we hope you do as well.
Second, let your son know that kids are going to pick on stuff like that and to just ignore it. If he likes death metal, then that is his thing. He is going against the grain (that being rap/etc) and being himself. Do your best to embrace his choice in music, as a rocker myself, I know some of the metal can be a little...well...noisy. Help him with his guitaring. Perhaps introduce him to some musicians (good ones, safe ones) in town. That could help him branch out into more forms of rock (quieter forms
) and learn more about guitar playing and of course make friends.
Third, if your son doesn't right away think WrongPlanet is for him, let him know that it is there and we won't judge, we won't be mean and we are here as friends cause we have Aspergers, Autism, ADHD and the like too...and some of us as rockers as well
Hope that helps and hope to see you and your son on WrongPlanet again soon 