The survival of William IX, count of Poitiers, would have a huge impact on the Angevin Empire. This makes him the firstborn of Henry II and Eleanor, making him the likely heir of the Kingdom of England. This also means Henry II has not four but FIVE sons he will have to give inheritance : William, Henry the Young, Richard Lionheart, Geoffrey and John Lackland.
Henry II will thus probably divide his Empire differently. William, as the firstborn, is likely to be given England, and maybe even Normandy. Henry the Young could get Greater Anjou (Anjou, Maine, Tourraine) and Richard could still become Duke of Aquitaine since he is Eleanor's favorite. If Geoffrey still marries Constance of Britanny, he will probably still get Britanny.
There arises the question of Poitiers... It is part of Eleanor's inheritance (She was both Duchess of Aquitaine and Countess of Poitiers), but was apparently already given to William before his death OTL. Thus, this could lead to a dispute between Richard and William over the County if William were to survive.
The question of John's inheritance also remains. OTL, John got the Lordship of Ireland but his father had to conquer it and it half failed. And when he tried to give more lands to John, his other sons rebelled. With another son, Henry II will find himself in a more complicated situation.
The Capetian strategy of "divide and rule" could also be altered in this scenario. OTL, Louis VII in 1173 then Philip Augustus through most of his reign turned the Plantagenêts against one another. With another Plantagenêt, they will keep the main goal, but they will have to take William into the equation.
A lot of the things to come will depend on William's character : he could very well be Henry II's favorite son (as John was OTL) or rebel against his father's despotism (as did his other brothers OTL). I tend to go for the second as the division of the Empire will always be a thorn in Henry II's side : his deals never satisfied his sons and it probably will stay the same in this scenario.