Forgetting to steal in Settlers of Catan
There are two issues here, and I think they each deserve a separate answer:


If you forget to steal, do you forfeit your right to steal a card?
If you go back a step in the game, do you re-take each step including rolling the dice?


The first is personal preference, I would say he forfeited his right to steal. It's not just he who missed the steal, it was the other players as well. It could be that people's hands have changed because they've received resources from the dice roll, and you'd have to account for that. In that, you would need to rely on everyone's honesty about (and memory of) the cards they'd received.

Re-rolling the dice after backtracking is something I never do, as a rule. If there's a possible argument about whether or not the dice should be rerolled, it's better not to, to avoid all discussion. No one is impartial in a game of Settlers.