As long as he is enjoying the reading, allow it.
The better the reader the better the student, they eventually really do get tired of these type of books. Do not make it a big deal.
Heck I would read a chapter and then our daughter would read a chapter of capt underpants.
I read every book our daughter ever read till sh was in 8 th grade. She loved discussing them. After that I could not keep up with all of the books she read, but we continued to share the reading all the way through her high school years.
I was and still am great friends with all of her former librarians. I used to tutor boys that struggled with reading and comprehension.
Goose bumps, capt underpants, tintin, all of those types of books are a gateway for many boys, to learn to like and enjoy reading.
I do not like schools banning books. That just makes the kids seek them out. Instead, I wish parents would read the controversial books along with their children and then have discussions about them. Children do understand fiction. Do not underestimate their common sense and intelligence. Banning books, means we do not trust our children to know the difference in behaviors. If you have done a good job with your beliefs and standards, your children know the difference than any book can .