Maybe someone can clear things up as I've not played online in quite a while, but could this be what's called 'lag switching'? I know it's relevant in other games online, and seems feasible to suggest it could have been used here - which is essentially a way of creating lag for the opponent while you can still play the game, thus gaining a massive advantage. I believe that's what it is anyway.
Maybe check out his game record and if he's got a ridiculously good record then chances are he's probably cheating. I'd like to think optimistically and say it could have just been a genuine fault in the connection online, but I'm not so sure. It's worth checking his online Win/Loss record before attempting to take action though as I say.
As for reporting him, it's easy enough, whether he'll be banned is another matter. Are you on the Xbox 360 or the PS3 by the way?
PS: Welcome to the forum.