Ditto the testing; he may have a reading problem that can be corrected quite easily.
I’ve always been a reader; don’t know why. My parents read to us (adult books) even when we were young. The first book I remember “reading” is Watership Down – I was younger than your grandson when it was read to me. Also, we had strict bedtimes, but were permitted to stay up 15 minutes later if we were reading. At his age, I think it was like a 7 or 7:30 bedtime, which was extended for reading only if I were fully ready for bed and in bed at bedtime, then I got to read in bed for 15 minutes. As a kid, it was totally worth it, so all of us would rush to be ready for bed on time just to get to stay up late!
Also, we were very restricted in our television watching. We lived in England in the 70s/80s, so there really wasn’t much in the way of choices of things to watch on TV the way there are now (I think we got only three or four channels). Nevertheless, while I lived in my parents’ house (i.e., until I turned 18), we were permitted one hour of television a day on schoolnights, and one and a half hours of television a day on weekends. All the kids had to agree on what to watch; if you didn’t want to use “your hour” to watch something someone else was watching, you had to leave the room. This was, of course, mostly before videos. Once we had the capacity to watch videos, they counted as part of our tv watching (unless the 'rents gave us special dispensation, which, as I recall, we got for things like special occasion showings of Wizard of Oz, Sound of Music, etc.)
The other thing that I recall is that my parents read. A lot. So we saw that around us all the time, and dinner table conversation revolved around what they’d read in the paper, magazines, books – it was all around.
What does your grandson like to do? Perhaps you can find books or magazines at or near his level that fit with his interests, like video games, or animals, or sports. I also recommend seeing if you can read to him more. He may be feeling pressured about the reading himself, but if you read to him then, as others suggested, he may become more interested in picking up the books himself. Best of luck.