Your grandson’s stuttering may have been caused by brain damage but chances are, it’s a coincidence that he also stutters. Does anyone else in his family tree stutter? If so, then it may be genetic as current research shows compelling evidence of genetics being involved.
No matter the cause, what is important is your grandson’s quality of life. And as a person who stutters myself (I also produce a lot of digital content about it), I can attest that your grandson is living in an age with A LOT of stuttering communities online. Along with @aliskahan‘s recommendations, I’d also suggest introducing your grandson to the National Stuttering Association http://www.westutter.org/ (I attend their conferences and they are a lot of fun) and/or FRIENDS (http://www.friendswhostutter.org/). There is a lot of misinformation about stuttering online but these three are great, reputable sources. He isn’t alone in stuttering and he can certainly live a full life and do everything he wants to do in spite of stuttering. It’s not how you say something, it’s what you say that is important! Those two organizations (and the Stuttering Foundation) provide positive role models of all ages.
As for being able to sing and read a lout without stuttering – this is very common since, as I understand, those two involve a different part of the brain that doesn’t deal with communication.