Children with learning difficulties sometimes do not benefit from being retained but would do best in staying with their peer group and receiving added academic support. If your child cannot read at 6, holding him back may not be a bad idea if his entire school performance is suffering. If he does well in other areas, it may just make him bored. Take him to the library and allow him to pick a book he wants to read. Comic books can sometimes motivate a child who is not generally interested in other books.
Most schools also have testing available that can identify if a child is perhaps dyslexic.
Reading is extremely frustrating for kids who have a minor correctable problem. A tutor at school who can spend some individual time may help. I also suggest a trip to the eye doctor. It may be something as simple as needing glasses to better see the words. If your son cannot read at all, then it might be better.