If he has indeed taken off the stickers, scuff sheet and extra grip and is complaining that the bat is now 2oz lighter, then the man is an idiot.
Likewise, if it has spent those 3 weeks next to a radiator or boiler and he's complaining it's now 2oz lighter, the man is an idiot.
If on the other hand it has just lost 2oz from drying out 'naturally' it would be fine to ask a question about it; for all he knows, you may have been keeping it somewhere damp and now he's taken it inside, it's dried out, and hence is now lighter.
I think you should establish what he's done with the bat (if anything) and where he's been keeping it. This may enlighten him.
You have done your due diligence as a seller, really. (Unless you had been storing it in the shed, mind!)