CapnZapp
Legend
I think a more direct point can be made by saying:Start by asking what the Druid wants. I mean, obviously they want something they can't get back at the druid grove, otherwise why would they have left to become an adventurer? You shouldn't assume that every Druid is a selfless devotee with no personal goals beyond the advancement of their faith, any more than you'd assume that of Clerics or Paladins. So besides the ideas that have already been offered...
Do they want revenge? Maybe someone burned down their grove, maybe someone killed their father, revenge tales are an ancient classic. Well, who says they have to do it themselves? Maybe they're putting bounties on the outlaws that did them wrong, setting the hunters of men to do their dirty work.
A natural life is not a monastic one. Druids don't have to live on roots and berries. In fact, what better way to enjoy nature's bounties and share them with others than by throwing lavish public feasts? They're a way to impress the masses, they give you an excuse to spread coin to hunters and farmers that follow your faith, and damn if you don't love a good roast boar.
Nature's wisdom means nothing if it isn't put into practice, and that means getting it into the ears of powerful and influential people. And if they won't come to the grove, well, you'll go to them. I mean, obviously not in person, that'd take forever. But you've written a set of scrolls distilling your philosophy and offering ideas on how it can be applied by city folk without abandoning their cities. Now you just need to hire scribes to make copies and send them around with thebribesgifts to get them into the right hands.
Assuming that every Druid is a selfless devotee with no personal goals beyond the advancement of their faith...
...is entirely fair and okay EXCEPT IT DOESN'T WORK in the present campaign.
The onus is on you the player to come up with a character concept that fits the campaign style.
It is the idea I just show up with my special snowflake of a character and it's the DM's job to work that in somehow that is the core problem.
So while I appreciate you trying to reason with the Druid player, I think it's much faster to simply agree some Druids ARE selfless, and then get straight to the point:
"That character doesn't fit the campaign, please make up another"
After all, he doesn't HAVE to play a Druid this time.
Or, to be more precise, he's definitely not entitled to play whatever he wants. It's his responsibility to create a character that will be fun to play. Not ours, as the DM.
Tldr: if your idea of a Druid is selfless, idealistic with no material needs... then don't play one in xp for gold. Simple! [emoji847]