He has no children. His heir at this point is his brother Augustus William, who died two years into the war in OTL. At that point, the throne goes to that brother's son: the very same guy who inherited it in OTL later on, Frederick William II. But that one is like 12 when the war starts, and 14 when his dad dies. So who gets to be regent? Probably Prince Henry, who is credited by some historian as more militarily capable than Frederick II! And it is often believed that henry had a huge chip on his shoulder, because Frederick II was a bit threatened by him and deliberately kept him from gaining as much gloty as he desired. enry, as regent, might just try to do something really bold to prove himself the greater military leader.
Of course, he might also just decide that as regent, he's not allowed to make such beg decisions. Who knows? But it's pretty certain that Augustus William would be far more cautious and hesitant in 1756-1758 than Frederick was in OTL, and then when he died in '58, Regent Henry would at least be personally inclined towards decisive action and bold decisions.