I thought he was annoying, but I don't see why you would say he can't sing. He sounds pretty good to me, especially considering this was done through a crappy computer mic with no vocal processing whatsoever. He was a tiny bit pitchy on a few notes, but overall, he sounded pretty good to me. I'm not a fan of that music style at all, but he definitely "can sing."
I really hate it when people say things like someone "can't sing." Where do you go with that? I guess it's your opinion and all, but what do you mean by "can't sing?" Can't sing in tune? Wrong, his tuning is pretty good for the most part. Don't like his tone? Ok, well that's your opinion.
I think it would be more accurate to say that you don't like his voice. That's a simple opinion, and there's nothing disputable about that. But saying someone "can't sing" is stating something as a fact. A fact is supposed to be proven or not, and there's no way to prove or disprove whether someone "can't sing." Unless maybe if they're incapable of making a sound at all. Then I guess you could say they "can't sing."