he was ONLY doing that to test me by buying this package of condoms.
?????????????????
That answer is so unlikely it's almost laughable.
Not that your situation is not serious. I am so sorry you are dealing with that mess.
But let's face reality, the most likely answer is usually the easiest answer. He bought the condom to to have sex. I'm not sure what confronting the OW would gain. She will lie too.
I think his answer is ridiculous though, fwiw.