Idioms and other expressions that point to the farm / countryiside origins of a person that now lives in an urban setting

You can take the boy out of the country, but you can't take the country out of the boy

This expression means that people who are from the countryside can leave, you can try to change them, but they will always stay true to their country roots.

More precisely this is a snowclone of the form “you can take X out of Y but you can’t take Y out of X”, found for example in A Dictionary of American Proverbs and The Dictionary of Modern Proverbs. This means you will frequently see another word, as appropriate, in place of “boy”, such as “girl”, “farmer”, or “man” (and the same is true for “country” though those wouldn’t be the expressions you’re looking for).

He just fell off the turnip truck

This idiom points to naivete and gullibility. The turnip is a rural vegetable, and implies that he has recently arrived from a rural region.

fall off the turnip truck: (chiefly US, idiomatic) To be naive, uninformed, or unsophisticated, in the manner of a rustic person.

[Wiktionary]

Although the OP has dismissed bumpkin in the comments, s/he might consider

country bumpkin

The idiom means:

An unsophisticated person from the rural area of a particular country. [Wiktionary]

The same entry has a number of good synonyms, some of which are more oriented towards being uncouth and others which are more close to nature (although all synonyms have an aspect of both).

1. boor (uncouth)
2. churl (uncouth)
3. hick (uncouth and close to nature)
4. hillbilly (uncouth and close to nature)
5. rustic (close to nature)
6. yokel (unsophisticated and close to nature)

Finally, I would be remiss to forget the word redneck, but it should be approached with caution, as it will almost certainly be considered pejorative when applied by an urbanite. It contains the notions both of being unsophisticated and as coming from a rural area. Please see this thorough answer.

Randy Newman wrote a song entitled Rednecks (1974). It calls out (more urbanite, more sophisticated) Northerners "hypocrites" for their denigration of Southerns (as rural unsophisticates). The pejorative "redneck" has been adopted as something of an anthem in the South, in similar fashion to the songs mentioned in the answer of the previous paragraph. The Malcolm Gladwell podcast entitled "Good Old Boys" discusses the evolution of the song's meaning. (Good Old Boys was the title of the album that Rednecks came from.)

I could offer son of the soil. It is not so much an idiom as metaphorical expression.

Here are some examples in context:

ludwig.guru son of the soil