Yeah, it's very unlikely I must admit
Good point about him possibly being Annie's son - we've got to find out what happened to her at some point, surely! Ben is still fond of her I would think, since he still has the doll set she gave him for his birthday all those years ago. However, I would tihnk that he'd be more loving towards Karl if he was Annie's (or his own for that matter).
I personally wouldn't be too keen on a Karl centric flashback, but perhaps seeing him as part of someone elses FB would be good.