Walnuts are toxic to animals, too.
I think it depends on how many cashews he ate and the type. If they were roasted and salted (they'd taste pretty awful raw, so I assume they are roasted), then you need to be concerned with how much sodium he consumed, too.
Two things to look out for: Vomiting and getting a nut stuck in his throat. Him not being able to pass a nut in his stools, because face it cats don't chew.

If he has no issues, do call the vet Monday and check his stools.