i dont think knowing makes any difference. no good can come from continuing to ask about an A. you'll have to believe/trust him and this is a good place to start.
forgive the person.
continuing to dig will only push him away, and any knowledge (if anything did happen) would only hurt you and give you something to use against him.
but if you do need to know, be sure you can handle the answer.
what if he admits to something one step more? will you push further? and when will you stop? -- once you start down that road there is no stopping. the only place you'd stop is when you've decided he's gone too far and the relationship is over.
"In a ham and eggs breakfast, the hen is involved, but the pig is committed".