It sounds like he has autism to me. I should know too. It might be that he has a type of autism known as asperger's syndrome, which is what my 16 year old son was diagnosed with when he was 7.
First of all, don't be afraid, it is not like the commercials they used to run where the child sits and shakes his hands and that is all he does, is pretty much unreachable otherwise. Does your grandson suddenly regain his wherewithal when something that he loves starts to happen? My son would act deaf until barney or shiningtime station's intro music would start. He'd run into the living room from two rooms away! It was so vexing to me at the time- i had his hearing checked out really thoroughly because autism didn't occur to me.i'd been watching the tv ads about it.
You have a good source of info at your fingertips. Just google either asperger's syndrome or autism, there are so many sights, and written in realspeak, not docspeak, so we can understand what we're up against.
My son has had a less than outstanding career at school ,and i blame the school's staff for that. On the bright side, which is what i want you to know about, he is astonishingly intelligent about mechanical things, and especially electronics. I mean astonishing for a college attending, 'normal' person, he's that good at it.
I now home school and dylan is thriving.
If i'm wrong about the autism, then i hope things get better and y'all find the solution. In the meantime you've learned about something that is on the rise and it's sure you'll run into it in your travels. If it turns out that he does have autism or asperger's, it's best to be armed, and start asap.
God bless! And god bless your grandson especially.