FWIW I'm inclined to agree with CoachJim and NextLevel: have you watched him bowl in league? Does he really know what he's doing? (You haven't indicated that.)
If he's bowling 3-4 time per week on TRUE heavy oil, all oil balls do absorb oil and lose reaction at different rates; he could be on REALLY heavy oil and they could be just sucking up the oil, if he doesn't wipe them down every shot and clean them after each session. But that seems unlikely, but not impossible, in just 3-4 weeks.
Have you examined these balls that no longer work? are they polished by oil? Have you tried to extract the oil by some method: rejuvenator, hot water, Hook Again, CPR?
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"...for advice is a dangerous gift, even from the wise to the wise...."
J. R. R. Tolkien