Maybe it has something to do with the core in 13 lb versions the respective balls.
The Gladiator Pearl is a MUCH stronger ball that the SlingShot. It handles true medium oil with ease, while the Slingshot cannot come close to handling medium oil. It hooks more an earlier on medium-light oil, while the SS would skid long, for the average revs, average ball speed bowler. On light oil the glad Pearl will just die in the heads, while the SS will look virtually like a power ball.
I'd suggest that Either your son is not seeing any decent oil amount at all, such that the SS conserves energy properly on this pattern, or the Gladiator has many, many games on it and it totally oil soaked. On such short, light oil, the Glad Pearl can easily be burning up in the heads and the midlane.
--------------------
"None are so blind as those who will not see." BowlingChat.net