Translation:Your son is always eating up all our strawberries.
I think the "opp" is there to emphasize that he eats every single strawberry until there are none left. You don't have to add it to the sentence, because it would also be correct without the opp. But if you think that "alle" is not enough emphasis, then add "opp".
And in that sentence the people who had strawberries want to sound as dramatic as possible, so they emphasize it more. ;)