I think his attraction to field service has a lot to do with a yearning to be accepted as "one of the boys". He's the youngest son of a half-dozen boys and three girls. Growing up, he was left to wander about the farmhouse and field in his nappy while his brothers tilled the fields. I am sure he did his best to tag along, but his brothers were busy with their own affairs, as big brothers will do.
So field service on his own wouldn't satisfy
Then let him know that his desire to "Serve Jehover" is not without ulterior motives. He is interested in socializing, not serving his gawd.