I saw lots of kids every night that I went last year. I even saw some as young as 4 or 5. It isn't really recommended for kids though but really it is your decision. If your 8 year old is old enough to know the difference between real and pretend and loves scary movies, he should be fine.
I'm a horror freak myself and I always loved horror movies and shows as a kid. I watched all the horror shows on tv like Thriller, Night Gallery, etc. and went to all the horror flicks at the movies I even made a haunted house when I was 8 or 9 years old for all my friends complete with spaghetti for brains and peeled grapes for eyeballs. I managed to grow up to be quite normal. I still love Stephen King books and they are a favorite of my teen sons also.
I would say that most 8 year olds would not be able to handle HHN but if your son is anything like I was as a child or like my own kids, he will probably love it. Maybe you could go one night without the kids and check it out first. If that isn't possible, then you could take him and if it seems to be too much, then ride the rides and go home.
edited to add: Be prepared to see plenty of fake blood and gore though. And just for the record my sons didn't go to HHN until they were 15 and 16.