If Jim is serving someplace as naturalist-in-residence, such as an ecolodge, then of course you'll need to pay whatever the ecolodge owner asks for lodging.

If you feel like giving Jim something for his time, please do, for Jim is not a rich guy -- as you'll clearly see -- and has no regular job. He does this because he believes in it. Period. You might give something beside money, too. Jim is a vegetarian who stays away from sugary stuff and heavily processed foods, but he'd surely enjoy something like some fruit, nuts or the like. Once you get to know Jim, you'll see ways you can reciprocate, if you feel that reciprocating is appropriate.