Without reading the article that went with this assignment, it's not possible to know what things were revealed in the article that you might ask him about. Here are some likely things to ask about in your note to him, though.
It's been a while since we talked. I miss that! I wonder what your job is like: are you working long hours? In all kinds of weather? Or do you get time off when it's raining? Do you have to provide your own lunch or does the boss brings lunches to the field for all of you? Can you get water to drink easily or do you take it with you in the morning? How often do you get paid? Is it a check or direct deposit? Would you recommend this job to me? I don't have a job lined up for the summer yet.
I'll be looking for your letter! Write soon, ok?
I'll be looking for your letter!