The best thing you can do is spend time reading with him, modeling an enjoyment of reading, and turn off the TV except for special times (like every Friday night or one hour per week or whatever rule you want to make). Same thing with video games and computer time.
Reading, compared to these activities, is very hard work for someone so new to it. And he's probably already feeling discouraged. So, he will always want to default to "screen time," instead. He needs to get through the hard work of reading, and it takes years for any child to get to the point where it really comes easy and feels natural. So, off with the electronics!
This sounds hard, but it's really the best way to do it. Marie Winn's book The Plug-In Drug (which is where I came to understand these things) offers some great tips on how to break the addiction. Highly recommended. And yeah. It can be hard. My husband and I still find ourselves watching too many movies in a month - sometimes two a week, or even more when we're really getting distracted from the many other things we can enjoy doing together. BUT. When we keep the screen off, we know we're adding more to the quality of our lives. And we get a lot more done :)
Oh, and speaking of third grade: a study showed that 3rd-graders with TVs in their bedrooms had significantly lower grades (or test scores? I forget which) than those who didn't. If he has one, get that TV out of there!