Helping you nephew to understand "Mr. Tambourine Man" might be difficult unless he has some familiarity with the 60s drug subculture.
Many songs of that era (e.g. the Byrds' "Eight Miles High" or PP&M's "Puff the Magic Dragon"), were double entendres, with one meaning for the regular people, and a special meaning for the initiates of the flourishing drug subculture. "Mr. Tambourine Man" is a perfect example of that. In my opinion, the whole song is an allegory of an LSD experience.
The song is filled with mages like "diamond sky" (think also of "Lucy in the Sky With Diamonds"), and the most obvious of all "Take me on a trip upon your magic swirling ship". Throughout the song, Dylan alludes to the effects of the drug itself, and the desire of its users to withdraw from the predominant culture which they loathed.
I believe Dylan used the tambourine reference in part because tambourines were briefly popular in the "folk-rock" movement, and also as an allusion to the pied piper story, with rock musicians playing the role of the piper.
My only regret is the Byrds chose not to record the entire song as Dylan wrote it, maybe to make it more palatable to the teenyboppers, and almost certainly to keep it short so it would fit on a single.
Whatever you might think about the drug connotations, some good and much bad, it's still a beautiful song filled with amazing imagery.
And Dylan was certainly one to play word games with his listeners, deliberately creating mysteries of meaning for people to pick apart, analyze, and argue over.
In the end, the meaning of a song like "Mr. Tambourine Man" might not be so important after all, as long as you join with it and lose yourself in the flowing images. And that, after all, is what poetry is all about.
Dylan was always a prankster. When he had his show on Sirius (he may still have, I don’t know) he used to pretend he had a vicious, running fight with Gordon Lightfoot. Then he would spin his favorite records.