I use twitter almost exclusively as a news feed. I will be the dissenting voice and say I think it's fine if 100% supervised at the age. You keep the password. Help him choose who to follow. Make it a protected account. Don't post pics. And regularly check if anyone is requesting to follow him.
If your computer is in a public location in your house, and if you monitor it regularly and choose what he follows, then I agree that twitter is harmless. All this hysteria about child predators is a bit over the top. I use twitter as a newsfeed myself, and never tweet.