It's always seemed to me that if you give a little information to someone about yourself, then they are very likely to confide information in you in return.
So maybe whisper him or something... and slowly direct the conversation to how he has so many flasks.
COMPLIMENT/WORSHIP/PRAISE him for it. Act impressed. People are much more likely to say something they won't usually if they see that it makes you think higher of them.
Again, approach the topic slowly, but maybe explain how you used to bot all the mats for pots in TBC times and had tons of pots back then too (In other words, create a common bond with him). Maybe ask if he has ever botted
in the past etc etc etc. I suggest you avoid asking him anything that if he responded would incriminate himself and his present account (because he will think you have bad intentions for asking all this and immediately shell up).
You want to build a bond of trust in him, giving him confidence that you are telling him all this secretive stuff... and hopefully in turn this trust will allow him to want to tell you anything in return.
Once he has admitted to botting (if he actually does) then you should immediately tell him you yourself bot (if you are fine with him knowing that). This should open up any type of info you want to get out of him.
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You can get just about any information out of someone if you really try. It's mostly about earning the trust in them. Be persistent, but careful. Also, be PATIENT. Approaching what you want to know rashly won't work.
Good luck