Yeah didn't even realize this was a thing. Seems like his dialogue should give a trade option.
You can trade with lots of NPCs that don't give that option in dialog. Just start talking to them, then move your mouse down to the trade button in the lower left. If the NPC has nothing to trade, then the button won't be there. Usually they have the same things you can pickpocket from them, like a health potion, a small amount of gold, and maybe something else. But Volo has a larger selection, and it gets recharged just like with the other vendors.