Firstly, do you spend a lot of time on the internet? As an older sibling you have to set the example. I know, I'm the oldest of four. Have you talked to your mom about the problem? If you try to rectify it but he goes above you then you won't be able to help him.
Take him outside and play games. Keep him interested and engaged in real life. Do this for a few hours everyday. Get him books! Books are great for young kids, they can be picture books. Eye Spy books are particularly entertaining for children.
When my brother and sisters were younger I made up games all the time to keep them occupied. We used to play one where I wrote riddles and hid them in various places, each riddle led to the next riddle, and at the end the first person to solve them all got some kind of reward. We played a weird catch game every time they had to clean their room. On road trips we played the "letter game" where you spell out the alphabet in order with the first letter of signs and billboards. Now that we've watched The Conjuring we've recently been playing hide & clap.
Or role play something, break out the nerf guns and pretend you're fighting zombies. Play charades. Teach him how to bake. Make friendship bracelets. JUST DISTRACT HIM. If you do it consistently you can ween him off the internet.
Are there kids he can play with in the neighborhood? Get them involved.
Seriously just go play. Explore. Learn something. He's bored with the monotony of early childhood, help him escape actively.