Did you explain to him it's a no no?

http://usbcongress.http.internapcdn.net/usbcongress/bowl/rulebook/2015-2016Rulebook.pdf

Chapter 8 Equipment Specifications
Bowling Balls
6. Holes
a.Holes or indentations for gripping purposes shall not exceed five and shall be limited to
one for each finger and one for the thumb, all for the same hand. The player is not required to use all finger holes in any specific delivery, but they must be able to demonstrate, with the same hand, that each gripping hole can be simultaneously used for gripping purposes. Any thumb hole that is not used for gripping purposes during the delivery would be classified as a balance hole.

It was brought up, and he pulled the whole "show me the rule" card.

By the rule changes that have been made the last two years, his ball is not legal for a no thumb bowler.  Two things have changed.  No weight hole allowed is the first. The second is not only for a two handed bowler, but for a no thumb bowler.  If a ball has a thumb hole, the ball must be weighed using the two gripping fingers as the centerline for checking the weight, not the center of the palm/grip in the conventional manner.  So without weighing a ball that has a thumb hole you would not know if its legal or not, but this no thumb bowler is not allowed a weight hole and a thumb hole.  Personally I dont care if they kick the ball down the alley, they still have to throw repeated shots.

Yeah, if you came at him nicely only to have him come at you with that attitude/response, I would definitely have the rule book waiting for him next time out. If you wanted to be a jerk, you could let him keep bowling like that and then have it called on him so that he has to forfeit. I probably wouldn't go that far, but it would be tempting if the guy is a douche about it.

In the last 10 years I have never had anyone reply cordial when they are told they are breaking or have broken a rule.  Many bowlers and people in general don't seem to care about rules.  I have had about 6 different incidents and they always have argued or look at me like I'm a big (fill in the blank with whatever you want) hole. 

Even when I called illegal pinfall on myself, when a pin hit some deadwood and then took out a pin on a split conversion, people looked at me like I was crazy for being honest and taking the count off my score.

Most of the time if people do know of a rule, they recite a rule that is 20 years old.  Most people do not keep up with the rule changes that happen over the years.

Also, with most rules USBC will not enforce the penalty if the person did not know they were in violation of a rule.  It is only after the person is informed, or it is known that the person knew they were in violation, where USBC will enforce the penalty.